The Doctor Angelicus is not in

From the Stanford Encyclopedia of Philosophy, under the "Homosexuality" entry; my bold:

The most influential formulation of natural law theory was made by Thomas Aquinas in the thirteenth century. Integrating an Aristotelian approach with Christian theology, Aquinas emphasized the centrality of certain human goods, including marriage and procreation. While Aquinas did not write much about same-sex sexual relations, he did write at length about various sex acts as sins. For Aquinas, sexuality that was within the bounds of marriage and which helped to further what he saw as the distinctive goods of marriage, mainly love, companionship, and legitimate offspring, was permissible, and even good. Aquinas did not argue that procreation was a necessary part of moral or just sex; married couples could enjoy sex without the motive of having children, and sex in marriages where one or both partners is sterile (perhaps because the woman is postmenopausal) is also potentially just (given a motive of expressing love). So far Aquinas' view actually need not rule out homosexual sex. For example, a Thomist could embrace same-sex marriage, and then apply the same reasoning, simply seeing the couple as a reproductively sterile, yet still fully loving and companionate union.

Aquinas, in a significant move, adds a requirement that for any given sex act to be moral it must be of a generative kind. The only way that this can be achieved is via vaginal intercourse. That is, since only the emission of semen in a vagina can result in natural reproduction, only sex acts of that type are generative, even if a given sex act does not lead to reproduction, and even if it is impossible due to infertility. The consequence of this addition is to rule out the possibility, of course, that homosexual sex could ever be moral (even if done within a loving marriage), in addition to forbidding any non-vaginal sex for opposite-sex married couples. What is the justification for this important addition? This question is made all the more pressing in that Aquinas does allow that how broad moral rules apply to individuals may vary considerably, since the nature of persons also varies to some extent. That is, since Aquinas allows that individual natures vary, one could simply argue that one is, by nature, emotionally and physically attracted to persons of one's own gender, and hence to pursue same-sex relationships is ‘natural’ (Sullivan, 1995). Unfortunately, Aquinas does not spell out a justification for this generative requirement.

It's a house of cards, I tell you....