An excellent point. Catholic teaching on birth control has long been ignored by both the people, and the heirarchy that is supposed to teach and preach it. Just about the only good decision that Paul VI, who presided over the disaster of Vatican II, ever made was that of reaffirming traditional Catholic teaching on artificial birth control. The problem was that the modernist heirarchy and clergy defied him and just let Humanae Vitae fall like a stone. Catholics haven’t been taught the sinfulness of artificial birth control for 40 years now.
If Catholics hadn’t taken to practicing birth control, Britain would have been Catholic a long time ago, and Islam would be nowhere near the threat it is now. I think things are coming back, though: after years of seeing one or two-child families in the pews, I’m seeing young conservative Catholic families with 4 or 5 children. So maybe Paul VI’s one good act will have some effect after all.