This is by far the most difficult of the posts to write because it comes with so many automatic objections. At any rate, what I'm going to try to do is lay out the main arguments here and answer the obvious objections in later posts. Again, please see PART ONE and PART TWO first before moving on to this part.
I have always been fascinated with how people use the word "perverted." In fact, I would love if someone were to trace the history of the word. At least through the English-speaking world anyway. What's interesting about it to me is that it seems to be a sort of threshold concept. That is, when even the most liberal people come across some thing that really shocks them, they will use the word "perverted." Or some modern variation of the concept: "that's ****ed up," for example. And, of course, what counts as perverted changes with time. Masturbation, for example, was pretty commonly seen as perverted for some time. In fact, for a lot of people, it still is. For the more libertine and progressive person, though, this is absurd. "Masturbation is natural!" they'll ignorantly proclaim. Things like incest and bestiality aren't quite there yet, but incest, I estimate, is getting there.
Because it does change over time, people make the assumption that the term "perverted" doesn't really refer to any objective thing. This ties in to what I said in the last post about emotivism. People, at least the more liberal ones, take the position that terms like "perverted" are similar to "good" and "bad." They merely represent an individual's preferences on the subject. But as I noted in parts one and two of this series, this is not the case for traditional morality. Traditional morality is based, as I noted, 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 I noted in part two of the series, the essences of living things are unique. Or, that 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.
I have not talked much in the other two parts about sexual organs or sexuality, though. I avoided it because it's the more controversial of the issues as it applies to morality. People can agree, for example that an eye is "for" seeing, at least generally. But they are way less comfortable saying a penis is for reproduction. I think they are unwilling to do this for a couple reasons. One, because they reject essentialism wholesale (without ever spending time with it), and two, because they don't want to even consider the fact that the thing they enjoy more than anything else in the world might be wrong. It's easy to get behind the idea that it's wrong to pluck out your eyes, but it's much harder to get behind the idea that it's wrong to use your penis sexually in a way that does not end in the ejaculation of semen into the vagina. But I'm getting a bit ahead of myself.
The Perverted Faculty Argument
The perverted faculty argument is based on the fundamentals of essentialism: that things have essences, that the closer a thing approximates itself to its essence, the "better" example that thing is, the more "real" 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 I 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 merely an extension of this. 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 necessarily be immoral because it would pull the thing away from its essence. In the case of eyes, it would be bad and consequently immoral to cut them so as to destroy sight. Sexual organs have specific final causes that are unlike eyes. The final cause of the penis, 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 for centuries (at least in the Western world): masturbation, contraception, homosexuality, sodomy, bestiality, etc. These things are positive frustrations of the final cause of sexuality: procreation. This is all that is meant by "perversion," and you can still see this meaning carried in how people use it in everyday terms. That is, a person perverts the natural end of an organ when he uses it contrary to its natural end. So, a person who has sex with animals is perverting his sexual organ's natural end, as his sexual organ's natural end is ejaculation into a human vagina, not into an animal. This is why people say "that's perverted!" or "he's a pervert!" Feser notes a wonderful quote from Freud on the topic. I don't repeat it here to justify the position (I know people hate Freud, and he wasn't exactly a natural law philosopher). I cite it just to buttress my argument that this is how the language has developed:
[I]t is a characteristic common to all the perversions that in them reproduction as an aim is put aside. This is actually the criterion by which we judge whether a sexual activity is perverse - if it departs from reproduction in its aims and pursues the attainment of gratification independently.
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. This is why the argument that masturbation is "natural" is completely faulty. "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 whatever), 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 is close to its nature or essence. While it absolutely is correct to say that masturbation or homosexuality occur in nature, it is positively incorrect to say that they are "natural" in the metaphysical sense. As such, when a philosopher or even the layman brings the charge against the homosexual that what he does is "unnatural" and the homosexual points to gay panda bears, he blindly misses the point.
The Two Aspects of Life
But isn't this different than an eye or a heart? I mean, 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 masturbation, we're not going to die. There are plenty of gay people that can attest to this. And something like HIV is the exception to this general rule. It's not like every person who masturbates or who has gay sex dies from it. You can "flourish" and still have gay sex, right? It's not like you're cutting up your eyes or drinking poison! So what gives. As I said, the final cause, or the essence of a body part is always determined in reference to what the body part is aimed. But the sexual organs are unique; they aren't concerned at all with survival; the are aimed at, instead, reproduction. There are two main aspects to the essence of life: survival and reproduction. Most body parts and body functions (at least for mammals) 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 celibate. What they're doing has nothing to do with the survival aspect of the essence of life, even though it does have something to do with the reproductive aspect.
As such, 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 exactly what Thomas 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. But in vain would be the generation of man unless due nurture followed, without which the offspring generated could not endure. The emission of the semen then ought to be so directed as that both the proper generation may ensue and the education of the offspring be secured.
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 I've said, it's a question of metaphysically impossible. 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 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 vagina is not doing that in that moment. Thomas 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. All her parts, 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.
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. This is not the case at all, and natural law would not require such things. 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. It is only a perversion when it is contrary. If a man thinks his wife looks ravishing in her new lingerie, and he can't hardly contain himself, he does nothing immoral having sex with her, even if he's not thinking about children at all. In fact, I hope he's really only thinking about how beautiful his wife is in that moment! (I envy him!) This, incidentally, is the justification for the enormously controversial Natural Family Planning, which, if understood in its proper philosophical context, isn't really controversial at all. (I'll do a lengthier post on NFP later on; this one is getting too long as it is.) Similarly, even if the homosexual 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 count as immoral.
I think this is a decent stopping point. There's a lot going on here, and there's a lot to tease out. All I wanted to do was get the basic formulation up and out of the way before I go into some of the nuances. I think it's a vital part of the blog in general. I want to emphasize that this does not represent the whole of sexual morality. It is merely the most important starting place. There are concepts of virtue and other aspects of human life that are absolutely essential to understanding what human sexuality is and how it should work. This is just the groundwork, the stuff that gave the Western world (and the Church) its baseline. Anyway, I'll talk with you all soon. I'm gonna be late for class!