Responsible parenthood . . . retains a . . . significance of paramount importance, which refers to the objective moral order instituted by God. . . . From this [husband and wife] are not free to act as they choose in the service of. transmitting life, nor are they free to decide for themselves what is the right course to follow. On the contrary, they are bound to ensure that what they do corresponds to the will of God the Creator . . . The Church . . . teaches as absolutely required that any use whatever of marriage must retain its natural potential to procreate human life ... it is a serious error to think that a whole married life of otherwise normal relations can justify sexual intercourse which is deliberately contraceptive and so intrinsically wrong."

Source: Paul VI, 25 July 1968.