The sexual activity, in which husband and wife are intimately and chastely united with one another, through which human life is transmitted, is, as the recent Council recalled, “noble and worthy.’” It does not, moreover, cease to be legitimate even when, for reasons independent of their will, it is foreseen to be infertile. For its natural adaptation to the expression and strengthening of the union of husband and wife is not thereby suppressed. The fact is, as experience shows, that new life is not the result of each and every act of sexual intercourse. God has wisely ordered laws of nature and the incidence of fertility in such a way that successive births are already naturally spaced through the inherent operation of these laws. The Church, nevertheless, in urging men to the observance of the precepts of the natural law, which it interprets by its constant doctrine, teaches that each and every marital act must of necessity retain its intrinsic relationship to the procreation of human life.
This particular doctrine, often expounded by the magisterium of the Church, is based on the inseparable connection, established by God, which man on his own initiative may not break, between the unitive significance and the procreative significance which are both inherent to the marriage act.
The reason is that the fundamental nature of the marriage act, while uniting husband and wife in the closest intimacy, also renders them capable of generating new life—and this as a result of laws written into the actual nature of man and of woman. And if each of these essential qualities, the unitive and the procreative, is preserved, the use of marriage fully retains its sense of true mutual love and its ordination to the supreme responsibility of parenthood to which man is called. We believe that our contemporaries are particularly capable of seeing that this teaching is in harmony with human reason. – from Humanae Vitae (1968)