Traditional sexual morality is concerned, above all else, with what a thing is. If one can determine what a thing is, he will be able to determine what should be done with that thing. Put another way, traditional sexual morality is concerned with what is called essence. And to understand what a thing really is, the four causes, developed by Aristotle, will help. It is important that "cause" not be read how you want to read it. Do not think of it in the "what caused the house to start on fire?" sort of way. Think of it more as the characteristics a thing has that make it what it is or do what it does. These are all tied to a "what caused the house to start on fire" sort of question, but they necessarily precede it.
There are four causes: material, formal, efficient and final. They will be taken in order. The material cause is the stuff a thing is made out of. In the case of a tree, it would be the actual material substance that makes up a tree: wood, bark, the atoms, etc. The formal cause is the form a thing takes to make it what it is. That is, it is the pattern or structure the aforementioned materials participate in. It may help to use an example showing the difference between matter and form, or material and formal cause, in the case of a rubber ball: "The matter by itself isn't the ball, for the rubber could take on the form of a doorstop, an eraser, or any number of other things. The form by itself isn't the ball either, for you can't bounce redness, roundness, or even bounciness down the hallway." The efficient cause is that which brings something into being. For a house, it would be builders. For a human heart, it would be reproduction and the various biological processes required during human development. This is what most people mean when they say "cause" today. Why did a bowling ball just fall on to the dining room table? Because someone dropped it from upstairs.
The final cause is the direction or range of effects to which a thing is pointed, its inherent power. It is generally described as being the "purpose" or "goal" of a thing, whether with an actor intending an end or not. The best way to think of final cause is to think of it in relation to efficient cause. Thomas Aquinas writes that "Every agent acts for an end: otherwise one thing would not follow more than another from the action of the agent, unless it were by chance." That is, if a thing didn't have a particular final cause (didn't have some end toward which it were aimed), then efficient cause wouldn't exist; that is, a particular result would not occur. This intuitively makes sense. If a ball did not have as its final cause the inherit power to roll, you pushing it wouldn't result in it rolling. This applies to everything, including sexual organs. If the penis weren't aimed at ejaculation, it wouldn't ejaculate. And if sperm weren't aimed at fertilizing eggs, it wouldn't fertilize eggs. Scientists cite things like "DNA" as "blueprints" for natural phenomenon all the time. What they are appealing to is final cause. That is, the final cause of a particular set of DNA is a specific range of results. If the DNA didn't have this specific range of results, a particular characteristic in the living thing would not occur. It is important to note that final cause does not require deliberated intention in the sense that many understand purpose. A thing need not be conscious to be pointed at an end. Gasoline is pointed at fire (has that as a final cause), and fire will result if a match is dropped on it (an efficient cause actualizing the inherent power in the gasoline), despite the fact that gasoline clearly is not conscious.
Emotivism and Traditional Morality
Now that there is a broad foundational metaphysics in place, it is time to turn to how that metaphysics might instruct us in the question of morality. The modern world of believes that "good" is a value judgment. That while there are facts, there are values, which are a different sort of thing. And further, because "good" is a value judgment, it is necessarily subjective. I may say that charity is "good," but it's not objectively good, or so it goes; in the same way I may say a particular painting is good, but in reality, it's not objectively good. It's merely a matter of subjective taste. There are two popular ways to fix this problem, which can be called "emotivism." One is to ignore it, to fully subscribe to claim that there are no objective goods and bads, and two is to introduce God. That is, since the world is necessarily meaningless and without value itself, having God as the rulemaker will fix the problem. That is, God places value on things, and so He effectively makes moral statements objective. In other words, "it's wrong to murder" gains objective legitimacy because God has made murder wrong; God's values are placed upon the world.
Traditional morality is nothing like this. Traditional morality, coming from the metaphysics outlined above, says that one, things have intrinsic goodness or badness to them, and as such, you don't need to appeal to God to either understand or articulate the standards that we can objectively discover. The basic argument goes like this: a thing is "good" insofar as it comes closer to its form or metaphysical essence, and a thing is bad insofar as it was further away from that form or essence, lacks the qualities of that essence. So, taking the example of a triangle, a triangle is a good example of a triangle when it has three perfectly straight sides and angles that add up to 180 degrees. Now, this isn't a subjective judgment of triangles. It's an objective judgment because that's just what a triangle is, that's just what it means to have the essence of a triangle: to have three straight sides that add up to 180 degrees. In other words, it's not the viewer's subjective opinion about triangularity that's driving his evaluation of triangles. It's the objective essence to which he is appealing.
The same goes for anything, including living things. A lion is a good example of a lion insofar as it comes close to its lion essence. Lions are things that have four legs and strong jaws and are fast and smart enough to catch prey. A lion is a defective or deformed or sick lion insofar as it does not approximate its essence. So, a lion with three legs that can not move fast enough to catch prey is a defective lion. The same goes for humans. A child born with a cleft palete has a "birth defect" and one who cannot maintain his immune system is sickly. The whole of medicine is based on this principle. But the evaluation that "that is a good example of a lion" would be an objective one based on the essence of a lion, not on the subjective preference of the viewer. Philippa Foot, an atheist neo-Aristotelian philosopher, once said something like the only way to understand morality is to start with plants. All she meant was that goodness and badness are defined by a thing's essence or form. And a plant has the essence of life by having strong roots, etc., so a "good" plant would be one that had those characteristics, and a sick or bad example of a plant would lack those characteristics. This is the necessary starting point for evaluating goodness and badness.
Morality merely presents a special case of being able to choose the goods set up by our nature. Humans, because they have rationality, can see and understand what the essence of a human is and choose to approximate itself to that essence. In other words, a person can rationally choose the good, and he is moral insofar as he does so and immoral insofar as he chooses the opposite. That is, if I know it's good for me (objectively) to have teeth (it matches my essence as a human being) it would be bad for me to eat candy every day so that my teeth fall out. Even more extreme examples, I would be a good example of a human insofar as I have rationality to attain truth, so it would be bad for me to lobotomize myself. And, again, what would be good for a human is an objective issue. It doesn't matter what a particular human might feel is good for him. So, an alcoholic might personally think it's good to drink more than is healthy or safe, but in reality, it's actually not, based on what he is as a human being. Even if he's predisposed to loving alcohol, it wouldn't change this evaluation.
Now, for living things, the good is always defined in teleological terms. That is, there are specific final causes to which we are aimed as human beings, and our acting in a way that does not fulfill or flouts those final causes would be bad or immoral. (The virtues (prudence, justice, temperance, fortitude), which lie at the root of most traditional moral standards, are those things that allow us to fulfill the final causes built into us by our nature.) An eye is for seeing, so a person has good eyes insofar as his eyes see well. Legs are for moving, so a person has good legs insofar as his legs allow him to move. But why are eyes for seeing and legs for moving? Though is may seem obvious, why is this the case?
Goodness and Life
As noted, a final cause should merely be understood as the direction to which a thing points based on its nature. So, rocks have final causes as things that roll because that's just what they are pointed at as rocks. In other words, if they weren't aimed in that direction, the result would not occur if a particular efficient cause actualized the inherent power in them (someone pushing them, for example). Now, I said that eyes are for seeing. But why aren't eyes for lots of things? After all, eyes could be used for any number of things: like being taken out and juggled. But how are eyes different than rocks? After all, it would seem that eyes would not be able to be taken out and juggled if they didn't have that as at least one of their final causes, if this weren't a direction to which they were aimed.
Philippa Foot writes a great deal on this. She notes that living things present a special case (and Foot says we use a "special grammar" to describe them). Living things are concerned with "flourishing," to steal a term from Aristotle. There are certain ends a living thing must realize to flourish as the thing that it is. In other words, there are certain final causes that must be fulfilled in order for the living thing to more closely approximate its essence as a living thing. And the essence of a living thing is to maintain life and make other things like itself which likewise maintain life. So, taking something like the lion again: a lion needs strong, sharp claws to hunt and kill its prey to survive. If it didn't have strong, sharp claws, it could not flourish, it could not survive and it could not match or approximate its essence as a living thing. It would be appropriate then to call a lion without these traits "defective" or a bad example of a lion.
When discussing living things, the lower level of the living thing is always pointed to the higher level of the living thing. In other words, every part of a living thing is evaluated in reference to the organism to which the part is related. Imagining a lion were blind, it would in a sense be right to say that the final cause of the genes that made the eye blind were fulfilled (otherwise the lion wouldn't have been born blind), but it would be wrong to say that the lion's essence as a living thing were fulfilled, as sight is aimed at (and likely required for) survival, and consequently flourishing, and this survival is required to approximate the essence of a living thing. The genes that control eyes are necessarily metaphysically subordinate because of the thing that an eye is. That is, even assuming that the gene's final cause that made the lion blind were fulfilled, that gene is still related to the eye, a thing which has as its essence sight, something that allows the flourishing of the greater living thing. All of the parts of a living thing are necessarily pointed upward, or to use a different verb, pulled upward by the final cause of the living thing (or of the essence of the living thing) in question.
So, when one says "X is the final cause of the Y part of the Z living thing," he is really saying, "X is the final cause of the Y part that counts as relevant to the Z living thing because it allows the Z living thing to flourish as the thing it is." In other words, a living thing's parts are metaphysically aimed at the flourishing of the living thing in a way that is not the case for non-living things. There are certain final causes that matter to living things that do not matter to non-living things. This is why a sentence like "his lungs are defective" can make sense, while a sentence like "that rock is defective" cannot. The lungs are defective in reference to the larger living thing's essence, while the rock has no thing to be in reference to. It is just itself as a rock. This what Foot was getting at when she mentions a special grammar. And this is all that is meant when a scientist says that a plant grows upward in order to get more sunlight. They are not saying that the plant is deliberating, and they are not saying that some outside force like God designed it this way. They are merely saying that this final cause of whatever in plants that makes them grow upward allows the plant to flourish as the thing that it is, to more closely approximate its essence as a living thing.
Again, there is no need to appeal to God for such an analysis. This goodness is built into the living thing by its very nature. This is what Foot calls "natural goodness." As Michael Thompson, another neo-Aristotelian philosopher, writes, "And so...even if the Divine Mind were to bring a certain life-form into being 'with a view to' securing an abundance of pink fur along the shores of the Manongahela, this would have no effect on the natural-teleological description of that form of life." In other words, the essence of life is a thing unique to living things that, even if God were to have designed the world, only the designs that allowed that living thing to flourish as a living thing would be relevant to that determination of what a particular part of a living is "for." This is why it makes sense to say that random spots on a lion tongue aren't for anything, but that having strong, sharp claws is for flourishing, and thus the strength or sharpness of claws can be analyzed as "defective" or "bad." One is about the essence of life, and the other is not, regardless of what God has done or will do.
The Perverted Faculty Argument
Traditional morality is based on the premise that things have real essences, that there is such thing as a triangle, and thus, triangularity, that there is such thing as a eye, and thus, the essence of eyeness. As likewise noted, the essences of living things are unique. Or, put differently, the final causes of particular body parts or body functions are to be determined in reference to the organism to which the body part is a part of. In other words, an eye has as its final cause sight (and various other things) because sight (and those various other functions) are necessary to the flourishing of the animal, are necessary for the animal to have the essence of a living thing. The perverted faculty argument is based on the fundamentals of essentialism: that things have these essences, that the closer a thing approximates itself to its essence, the "better" example that thing is. As noted, this is an objective sense. A lion that can catch prey, protect itself, run, move, etc. is an objectively good example of a lion, while one that has three legs or is blind in its left eye is an objectively bad example of a lion. Morality, as noted, is merely a special case that applies to rational agents. Insofar as the rational agent chooses to approximate itself closer to its essence, closer to goodness, the agent is moral.
The perverted faculty argument is simply an extension of all of these principles. It says that because essence is defined, especially in living things, by final cause, to frustrate or to act contrary to that final cause would be immoral because it would pull or position the thing in a way not aligned with its essence. It would be to misuse or pervert the faculty, which, by the nature of things in question, could never be good. The final cause of the penis in the context of sexual activity, for example, is ejaculation into the vagina. This is the direction to which a penis, by its essence, is aimed for the flourishing of the human being. As such, it would count as immoral to use a penis in such as a way as to act contrary to its final cause; namely, it would be immoral to ejaculate into something that isn't a vagina. To do so would be to willfully choose an end that is not a good (in the metaphysically objective sense) end. It would be to choose bad. If this is correct, that acting contrary to a thing's end is immoral, then it would rule out the things that have been considered sexually suspect by many people (at least in the Western world): masturbation, contraception, homosexuality, sodomy, bestiality, etc. These things are positive frustrations of the final cause of sexuality: reproduction. This is all that is meant by "perverted" in the "perverted faculty." That is, a person perverts the faculty (or process used by the organ) when he uses it contrary to its natural end. So, a person who has sex with animals is perverting his sexual organ's faculty, as his sexual organ's natural end is ejaculation into a human vagina, not into an animal.
This is also what people mean when they say homosexuality is "unnatural." They are saying homosexual activities run contrary to the final cause of the sexual organs and sexuality in general. The same goes for masturbation. "Natural" in the natural law sense is concerned with metaphysics. An eye by its nature (or essence) is aimed at sight for the flourishing of the animal. Even if every person were to be blinded tomorrow (by disease or what have you), it would still be the essence of the eye to see. Similarly, even if there are people or animals who are born blind, it would not make eyes, by their nature, things that do not see. In other words, "natural" in this sense does not mean "occurring in nature" or anything of the sort. Natural law does not say that something is good because it occurs in nature. Natural law says that something is good insofar as it aligns with its nature or essence. While it absolutely is correct to say that masturbation or homosexuality occur in nature, it is incorrect to say that they are "natural" in the metaphysical sense.
To quote Feser in summary of the perverted faculty argument: "The basic idea is that when some faculty F is natural to a rational agent A and by nature exists for the sake of some end E (and exists in A precisely so that A might pursue E), then it is metaphysically impossible for it to be good for A to use F in a manner contrary to E. For the good of a thing is determined by the end which it has by nature. F exists for the sake of E, and agents like A naturally possess F precisely so that they might pursue E. Hence (given the underlying metaphysics) it cannot possibly be good to use F for the sake of preventing the realization of E, or for the sake of an end which has an inherent tendency to frustrate the realization of E." To apply it to the reproductive faculty then, the reproductive faculty (F) exists for the sake of reproduction (E). As such, it would be metaphysically impossible for it to be good to use the reproductive faculty in a manner contrary to reproduction. Accordingly, two people of the same sex using their sexual faculties with one another must be unnatural in the above sense since it is by its very nature unreproductive, and if such a thing is chosen willfully, it must count as immoral.
The Two Aspects of Life
But isn't this different than an eye or a heart? To use our heart contrary to its end (pumping blood toward our survival) would be to kill ourselves. But when we use our sexual organs in perverted ways, like same-sex sex, we're not going to die. There are plenty of gay people who can attest to this. And something like HIV is the exception to this general rule. As noted, the final cause, or the essence of a body part, is always determined in reference toward which the body part is aimed. But the sexual organs are unique; they aren't concerned at all with survival; they are aimed at, instead, reproduction. There are two main aspects to the essence of life: survival and reproduction. Most body parts and body functions are concerned with the former, while only a few are concerned with the latter. A person might have sex every day, while another might be completely continent. Their activity has nothing to do with this survival aspect, even though it does have something to do with the reproductive aspect. It is the relevant aspect that guides the analysis.
Even though it is correct to say that the sexual organs are aimed at the exchange of fluids, it is even more correct to say that the exchange of fluids is aimed at the creation of life. In other words, to act contrary to our sexual ends would be to act in a way that is metaphysically contrary to the creation of life, what the sexual organs ultimately aim at. This is what Thomas Aquinas meant when he wrote:
Now it is good for everything to gain its end, and evil for it to be diverted from its due end. But as in the whole so also in the parts, our study should be that every part of man and every act of his may attain its due end. Now though the semen is superfluous for the preservation of the individual, yet it is necessary to him for the propagation of the species: while other excretions, such as excrement, urine, sweat, and the like, are needful for no further purpose: hence the only good that comes to man of them is by their removal from the body. But that is not the object in the emission of the semen, but rather the profit of generation, to which the union of the sexes is directed.
Hence it is clear that every emission of the semen is contrary to the good of man, which takes place in a way whereby generation is impossible; and if this is done on purpose, it must be a sin. I mean a way in which generation is impossible in itself as is the case in every emission of the semen without the natural union of male and female: wherefore such sins are called ‘sins against nature.’ But if it is by accident that generation cannot follow from the emission of the semen, the act is not against nature on that account, nor is it sinful; the case of the woman being barren would be a case in point.
As noted, this impossibility is an issue of metaphysical impossibility. This means something that is impossible by the nature of the thing it is. So, an anus, metaphysically, is not aimed at the creation of life. As such, it would be immoral to ejaculate into an anus. The same goes for a hand. Or a mouth. Or a sock. Or a condom. A vagina, on the other hand, is absolutely aimed at the creation of life by its very nature. Even if a woman's body is not doing so in that moment. Aquinas writes about this above. He says a barren woman would be a "case in point." A man is not a woman, barren or otherwise. A barren woman is still a woman. Her sexual organs, by their nature, are aimed at her creating life in her womb. As such, to ejaculate into a woman who is barren would not be unnatural in the appropriate sense, and it would accordingly be completely moral. (See post on NFP for more information.)
Please keep in mind this does not mean that when a couple has sex, the only way to do so morally would be to intend to create children. All it means it that when a couple (or any person) has sex, they must do so in a way that is not actively contrary to the creation of life. Similarly, even if the homosexual person does want to create life by ejaculating into his partner's anus, the anus, by its very nature is not aimed at the creation of life. As such, it would fall out as immoral. Similarly, even though homosexual sex surely feels pleasurable for the same-sex couple, and so pleasure is in a loose sense a final cause of their sexual activity, as that pleasure would not occur if it were not an end in the body parts in question, their sexual faculties (which include that pleasure associated with those body parts) exist for the sake of reproduction. As such, the relevant final cause, the final cause associated with the flourishing of the living thing, is in direct conflict with such a misuse, regardless of how pleasurable it may be.
"Other than" Uses
Would this entail that every time we use our mouths to chew, we must do so in a way that's nutritive? After all, the mouth is part of the consumptive faculty, which exists for the purpose of chewing food so that we can consume it. That is, the mouth has as its final cause the chewing of food for nutrition. As such, wouldn't this mean chewing gum (which isn't about getting us nutrition) or going to wine tastings (which involve spitting out what would give us nutrition) would fall out as immoral since they aren't doing that? The answer to these questions is no. The natural law does not say that a person acts immorally by using a faculty other than its natural end, but merely when using a faculty contrary to its natural end or in a way that necessarily frustrates its natural end. To understand why the above do not do this, we must determine their ends, as it is the relevant ends that guide the analysis.
As noted, the majority of the various body parts and faculties we have in our bodies, both active and passive, exist in order to keep us alive. We eat to stay alive. We see to stay alive. Our hearts beat to keep us alive. That is, keeping us alive (and healthy, as health is integral to that goal) is the aim of these parts and functions. So long as we do not act in a way that frustrates this purpose, keeping us alive and healthy, we use these faculties morally, at least as regards the immediate perversion of the faculty (they may be immoral for other reasons). When we do the opposite, we use the faculties immorally. While it is always prudent and often preferable to use these faculties in line with their immediate ends, as they are built in us for our flourishing and keeping us alive, frustration of the further end (survival and maintenance of ourselves) is usually more difficult and allows for much more flexible use (like chewing gum and wine tasting (though an argument could surely be made that wine tastings may be gluttonous and wasteful) or even removing a part for the sake of the whole). The sexual faculty does not have as its end keeping us alive, though. It has as its end the creation of life; the workings of the faculty are all aimed at this goal (arousal, lubrication, ejaculation, sperm, egg), similar to the way the workings of the other bodily faculties are aimed at keeping us alive. When we use the sexual faculty, then, in a way that is aimed at the creation of life, we use the faculty morally. When we do the opposite, we use the faculty immorally.
This would be true if you misused the sexual faculty today (wore a condom today during sex) but did not misuse it tomorrow (did not wear a condom tomorrow) because it is the act itself (having sex in a metaphysically non-reproductive way) that is contrary to its natural end, the creation of life. (This would of course not apply to something like chewing gum, which is not inherently or even accidentally contrary to the maintenance of the individual.) Such an argument would be similar to saying that lying on Thursday was not immoral because you told the truth on Friday. Similarly, it would not matter that the reproductive faculty were also for other ends: bringing two people closer together. If it is by its nature to misuse the faculty, using the faculty for another good end would not make that misuse moral. To make such an argument would be similar to saying that having cyanide at a meal with friends is a good use of the consumptive faculty because the consumptive faculty has social interaction as one of its natural ends.
To summarize then: with regard to the non-sexual faculties, the question is, "Does this use harm or destroy life?" If yes, the use is immoral. If no, the use is likely moral, save other considerations. With regard to the sexual faculty, the question is, "Does this use lack the power to create life?" If yes, the use is immoral. If no, the use is likely moral, save other considerations.