1.) Before 1960, all known forms of contraception were either barriers or poisons and so could be easily categorized as contradicting the (more or less biological) nature of sex.
2.) After 196o synthetic estrogen became widely available and could suppress ovulation, making pregnancy much less likely (failure rates between 2-10%). Unlike barriers and poisons, women’s bodies release estrogen to suppress ovulation all the time. So long as the only Catholic objection to contraception was its being a barrier or poison which interfered with a natural process, the Pill was licit.
3.) If there is something wrong with artificial contraception it could no longer be based on appeals to defending the integrity of a natural process. The easy condemnation of biologism was lost. What it was replaced with is articulated in Humanae Vitae pp. 12 ad 13, which argue that willful suppression contradicts love of one’s spouse (12) and God (13).
4.) HV speaks of the inseparable connection between the unitive and procreative significance of marital sex, and somewhere along the way the became the “dual ends of the sexual act”. The only point HV is trying to make, it seems to me, is that contraception is wrong because it is loveless, which is such a bold claim that everyone has been assuming that it must be saying something else. The point is not that marital intercourse has two autonomous goals that Catholics have to try to simultaneously both keep in view, it’s the much more striking claim that an essential condition of loving someone erotically is wanting to have a child with them, at least so far as this means doing nothing to suppress fertility.
5.) Claiming this does nothing to remove the usual objections. What about elderly marriage? Marriage to women with hysterectomies? HV might be content to just claim that the eros here can only be imperfect, though accounts of a type of thing don’t take peculiar circumstances into account. Just how far one can develop this idea of degrees of imperfect eros might be an interesting research project.