Sex expresses love, but John Paul II argues that more needs to be said ( Love and Responsibility ). After all, “There may be affection between people who are not sexually attracted to each other.” That suggests that “it is not love of man and woman that determines the proper purpose of the sexual urge” (51).

Rather, the end of sex per se “is something supra-personal, the existence of the species Homo , the constant prolongation of its existence.” This doesn’t reduce sex to biology or survival; existence is, for John Paul, a weighty philosophical rather than a biological term. Further, reproduction is intended to be bound up with love: “Looked at more closely and concretely these two persons, the man and the woman, facilitate the existence of another concrete person, their own child, blood of their blood, and flesh of their flesh. This person is at once an affirmation and a continuation of their own love” (53).

Still, love between persons is not the end of sex, but rather that love is “shaped, channelled one might say, by that purpose and formed from the material it provides” (53). The end - the existence of the species - determines the shape of the love that pursues that end.

You can see where this goes. For John Paul, it’s part of an argument against “artificial” contraception. But we can stop short of that conclusion and still say that it’s ethically relevant that sex is the means for producing new human beings and, on the other hand, that love can be deep and satisfying in the complete absence of sex. John Paul has done something intriguing here: On the one hand, he refuses to reduce sex to biology; more surprisingly, he sees a danger and a mistake in reducing sex to an expression of love .

In any case, the argument has obvious relevance to our current debates about same-sex marriage. Arguments for same-sex marriage link love and sex, while detaching sex from reproduction. An argument against same-sex marriage would have to demonstrate a linkage of all three, and avoid both reductionisms that John Paul identifies.

Once again we can see that the debate about same-sex marriage is not only about homosexuality but about the whole sexual regime of the past century.