Observing the Natural Law
11. 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.'' (11) 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. (12)
Union and Procreation
12. 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.
Faithfulness to God's Design
13. Men rightly observe that a conjugal act imposed on one's partner without regard to his or her condition or personal and reasonable wishes in the matter, is no true act of love, and therefore offends the moral order in its particular application to the intimate relationship of husband and wife. If they further reflect, they must also recognize that an act of mutual love which impairs the capacity to transmit life which God the Creator, through specific laws, has built into it, frustrates His design which constitutes the norm of marriage, and contradicts the will of the Author of life. Hence to use this divine gift while depriving it, even if only partially, of its meaning and purpose, is equally repugnant to the nature of man and of woman, and is consequently in opposition to the plan of God and His holy will. But to experience the gift of married love while respecting the laws of conception is to acknowledge that one is not the master of the sources of life but rather the minister of the design established by the Creator. Just as man does not have unlimited dominion over his body in general, so also, and with more particular reason, he has no such dominion over his specifically sexual faculties, for these are concerned by their very nature with the generation of life, of which God is the source. "Human life is sacred—all men must recognize that fact," Our predecessor Pope John XXIII recalled. "From its very inception it reveals the creating hand of God." (13)From: http://www.vatican.va/holy_father/paul_vi/encyclicals/documents/hf_p-vi_enc_25071968_humanae-vitae_en.html