To understand the unitive purpose we must recognize that the sexes are not only different but complementary. God could have mad just one self-sufficient sex. Instead, he made two, each of which feels itself incomplete and longs for the other. The canyon between them is deep, but bridging it is well worth the patience and discipline it requires.
To be sure, there are other ways to use the sexual powers, ways that do not bridge the canyon. For instance, solitary sex sinks a person more deeply in the self; sodomy sinks him into a looking-glass idol of the self. By contrast, marriage holds forth the prospect of altogether forgetting the self in care and sacrifice for the other. . . .
Notice how this reflection on the purposes of the sexual inclination has enabled us to distinguish between its natural and unnatural forms. That it can do so is Thomas’s point. Of course, when he says that everybody knows the natural law, he does not mean that everybody knows the theory; he only means that everyone has some intimation of the fact. Hence high-minded men of every civilization have recognized that marriage is sacred, but it does not follow that they know why.
J. Budziszewski, Written on the Heart: The Case for Natural Law, p. 71