For both Aristotleian and Thomistic NL, the telos of an existent is what informs the morality of its agency or use. But the argument as pressed against gay marriage fails because the Thomistic construction (or, at least as it appears often to be used) relies too heavily on biology as an indicator of teleology. To be sure, the proponents will deny that this is what they are doing. They will point to Biblical and Church-traditional rationales that support the ideal of a man-woman relationship as somehow metaphysically required or superior. By doing so they effectively deny any possibility of a metaphysical telos for the homosexual, summarily declaring him or her as acting outside of proper function. But if "proper function" is metaphysically, not biologically, determined, upon what rationale does their declaration rest? The only way they can justify it is to turn to some form of revelation, but doing so negates their claim to be basing their argument on NL.
John Corvino, the author whom Rowe cites, exposes this flaw in their reasoning by calling attention to the blatant inconsistency with which the Church will endorse non-procreative heterosexual sex but condemn non-procreative homosexual sex. As this forms part of a common argument one finds against gay marriage (from non-Catholics as well), it's good to see an explicit calling-out of this hypocrisy. Corvino writes:
Why the apparent inconsistency? Catholic natural law theorists answer that such acts can still be of “the reproductive kind.” But it is difficult to make sense of this claim, except as a lame attempt to deny unpalatable conclusions that clearly follow from the Church’s position. If a sexual act cannot result in procreation and the couple knows it, then how is the act “of the reproductive kind”?Indeed. "Lame" is an excellent word to describe this misleading and dishonest tactic. I agree with Rowe when he further points out:
In many ways, the Catholic position on sex is downright immoral and irresponsible. Even married couples can only handle so many children at once. Couples ought not bring more children into a family than they can responsibly handle. There are some couples, those on the right end of the fertility Bell Curve, who will literally have over a dozen children in a lifetime marriage. And for most couples, excluding the Bill Gates and Donald Trumps who could not only support all the children but hire as many nannies as needed to assist the wife, that’s just a bad, irresponsible life-plan to say the least.Catholics obviously rely on God to sort it all out; to ensure that children aren't born to families that cannot care for them. I'm sure that works just fine for celibate clergy, but over here in the real world family planning isn't just a good idea; it's a moral necessity. The telos of procreative sex need not necessarily include overpopulating the world and neither need the telos of sex itself be procreative.