The reception-sensus fidelium argument assumes that the acceptance or rejection of a teaching by the mass of Catholics reflects the action of the Holy Spirit. But not so long ago an overwhelming majority of American Catholics agreed that contraception was wrong. Looking at the numbers in 1963, sociologist and novelist Father Andrew Greeley, later a bitter critic, said Catholics "accept the Church's teaching with a vengeance."
Now, if "reception" and "sensus fidelium" were correct, we'd have to conclude either that contraception was wrong before the encyclical but acceptable after it or else that the Holy Spirit changed his mind. But both explanations are absurd. We need a better reason for the shift.
And in fact there is one – at least, in the United States. Starting after World War II, efforts began to bring about radical change in American attitudes on sex. This campaign included not only Planned Parenthood but foundations like Ford and Rockefeller and wealthy individuals like John D. Rockefeller 3rd, academic institutions, and elements of the media.
Some of the efforts were aimed at the federal government – successfully, with the Johnson and Nixon administrations pushing government promotion of birth control as a population limiting anti-poverty measure at home and abroad. And some was targeted at the Catholic Church, with collaboration from within by Catholic individuals and groups.
By the mid-1960s this campaign had converged with the development and marketing of "the Pill," an oral contraceptive that made birth control simpler than ever before, and with a raging cultural revolution – largely a sexual revolution – by then sweeping the United States and countries like it.
And so the stage was set for Humanae Vitae. After long delay and in the face of mounting pressure for change, Pope Paul issued his encyclical, only to be greeted by an immediate chorus of dissent. The encyclical didn't stand a chance.