Because that is far too narrow an interpretation of sex. There is "unnatural" about homosexual sex, and to say that procreation is the primary reason for heterosexual sex shows a profound misunderstanding of why people have sexual intercourse -- the sort of misunderstanding that only the celibate could come up with.

No, in this, as in most of Humanae Vitae, Paul VI showed that he didn't have a clue about what he was talking about. You want to see men denigrating women? Simply look at the Merrie Men of the magisterium with their belief that women are inferior to men.