 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.