EPTI think it's specious to suggest that, even within the confines of some sort of adapted form of natural law ethics, the procreative aspect of heterosexual sex somehow "saves" heterosexual promiscuity. To me, that just doesn't really make any sense, because I don't really evaluate ethical decisions based on things that really have nothing at all to do with the intent of the agent relative to himself. A promiscuous homosexual person engaging in sexual intercourse and a promiscuous heterosexual person engaging in sex are entirely equivalent, in a moral sense, from my point of view. To argue that the lack of a procreative element somehow damns homosexual sex more than heterosexual sex insofar as it's something like, I don't know, offensive to God (I'm not sure what else "corrupted from its intended use" could mean).
But even such a thing would beg the question: why do homosexuals exist, and, more importantly, why is their sexual identity so similar to that of heterosexuals? The way a heterosexual woman is attracted to men isn't particularly different from the way homosexual men are attracted to men.
But anyway, going back to the original argument concerning essences, you seem to sort of want to have your cake and eat it, too. On the one hand, you point to sex as being fundamentally natural, in some sense, and that only the human species enjoys other attributes like friendship, and so on. But on the other hand, you want to say that human beings, unlike animals (who quite clearly have all sorts of sex, including the homosexual kind) are, as moral agents, above animals and should refrain from "unnatural" behaviour. You try to allay this by saying that "reproduction is inseparable from the essence of sex." Aside from the fact that strict essentialism strikes me as a game of definitions, how exactly is reproduction any more inseparable from the essence of sex than the release of endorphins? Sterile, or, perhaps more realistically, post-menopausal heterosexual women, can engage in sexual intercourse with the reproductive element entirely removed; yet the release of endorphins, emotive qualities, and the ability to express love remain.
I'm not denying that procreation is central to heterosexual intercourse as an action; from a biological point of view, it certainly exists because it permits the transmission of genetic material. But at the same time, I see no reason to be confined to the principles of natural selection when making our moral codes. What exactly is the "natural" purpose of homosexuality supposed to be, then? It's pretty clear that that a lot of people are homosexual, and it's also clear that most of them have no associated pathologies (unlike other "sexually deviant" groups, like zoophiles, or pedophiles). Do you not think it, at the very least, a pretty cruel joke on the part of God to give these people the exact same sort of sexual identity as that of a heterosexual (with respect to how it feels to them, how they experience attraction/intimacy, etc.) but at the same time give them a body which, for whatever reason, forbids them from ever acknowledging or participating in that part of themselves?
If anything, the existence of homosexuality, along with the massive history of sex-for-reasons-other-than-just-procreation (both in humans and animals), is an argument against a procreative element being necessarily for moral sexual conduct.
I don't think unabashed Thomism really makes much sense in the context of a post-Psychological Revolution world. People aren't really as simple as things moving toward some end; sexual identity isn't really as simple as just "oh btw you like this gender," but rather impacts virtually every facet of a person's life.
I also don't think the conclusion of your Aristotelian argument really follows, either. If anything, it suggests that homosexual sex isn't actually sex, but is something else altogether, and it's not clear how that other thing would be immoral, given that, since it isn't actually sex at all, can't really be regarded as a corruption of sex. See, maybe it'd be better to think of the "essence" of sexual orientation (if such a thing exists) rather than the essence of sex. In that vein, it's not like homosexual people should be going out and having sex with the opposite sex (which would be a betrayal of their own integrity, and would show disrespect for themselves, at best) when they're not having homosexual sex.
In what way, for you, is homosexual sex similar enough to heterosexual sex to be considered "sex," but not similar enough to still be considered a corruption of the same? Is there another, purer (i.e. not corrupted) version of sex available to the homosexual?
