Either all Christians up til 1931 misuderstood this issue, or many Christian since 1931 misunderstand this issue.
Because of the general apostasy of our times, I'm betting on the latter. An age in which the Christian churches are caving on every moral issue in compromising with the world is not likely one in which they better understand God's Will on contraception. On the contrary, compromise on contraception is of the same nature as compromise on abortion, homosexuality, divorce and remarriage, etc.