No member of the faithful could possibly deny that the Church is competent in her magisterium to interpret the natural moral law. It is in fact indisputable, as Our predecessors have many times declared, that Jesus Christ, when He communicated His divine power to Peter and the other Apostles and sent them to teach all nations His commandments, constituted them as the authentic guardians and interpreters of the whole moral law, not only, that is, of the law of the Gospel but also of the natural law. For the natural law, too, declares the will of God, and its faithful observance is necessary for men's eternal salvation.

[…]

Married love particularly reveals its true nature and nobility when we realize that it takes its origin from God, who "is love," the Father "from whom every family in heaven and on earth is named." Marriage, then, is far from being the effect of chance or the result of the blind evolution of natural forces. It is in reality the wise and provident institution of God the Creator, whose purpose was to effect in man His loving design. As a consequence, husband and wife, through that mutual gift of themselves, which is specific and exclusive to them alone, develop that union of two persons in which they perfect one another, cooperating with God in the generation and rearing of new lives. The marriage of those who have been baptized is, in addition, invested with the dignity of a sacramental sign of grace, for it represents the union of Christ and His Church.

[…]

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.

[…]

Therefore We base Our words on the first principles of a human and Christian doctrine of marriage when We are obliged once more to declare that the direct interruption of the generative process already begun and, above all, all direct abortion, even for therapeutic reasons, are to be absolutely excluded as lawful means of regulating the number of children. Equally to be condemned, as the magisterium of the Church has affirmed on many occasions, is direct sterilization, whether of the man or of the woman, whether permanent or temporary. Similarly excluded is any action which either before, at the moment of, or after sexual intercourse, is specifically intended to prevent procreation — whether as an end or as a means. Neither is it valid to argue, as a justification for sexual intercourse which is deliberately contraceptive, that a lesser evil is to be preferred to a greater one, or that such intercourse would merge with procreative acts of past and future to form a single entity, and so be qualified by exactly the same moral goodness as these. Though it is true that sometimes it is lawful to tolerate a lesser moral evil in order to avoid a greater evil or in order to promote a greater good," it is never lawful, even for the gravest reasons, to do evil that good may come of it — in other words, to intend directly something which of its very nature contradicts the moral order, and which must therefore be judged unworthy of man, even though the intention is to protect or promote the welfare of an individual, of a family or of society in general. Consequently, 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.

[…]

It is to be anticipated that perhaps not everyone will easily accept this particular teaching. There is too much clamorous outcry against the voice of the Church, and this is intensified by modern means of communication. But it comes as no surprise to the Church that she, no less than her divine Founder, is destined to be a "sign of contradiction." She does not, because of this, evade the duty imposed on her of proclaiming humbly but firmly the entire moral law, both natural and evangelical.

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And now, beloved sons, you who are priests, you who in virtue of your sacred office act as counselors and spiritual leaders both of individual men and women and of families — We turn to you filled with great confidence. For it is your principal duty — We are speaking especially to you who teach moral theology — to spell out clearly and completely the Church's teaching on marriage. In the performance of your ministry you must be the first to give an example of that sincere obedience, inward as well as outward, which is due to the magisterium of the Church. For, as you know, the pastors of the Church enjoy a special light of the Holy Spirit in teaching the truth. And this, rather than the arguments they put forward, is why you are bound to such obedience. Nor will it escape you that if men's peace of soul and the unity of the Christian people are to be preserved, then it is of the utmost importance that in moral as well as in dogmatic theology all should obey the magisterium of the Church and should speak as with one voice. Therefore We make Our own the anxious words of the great Apostle Paul and with all Our heart We renew Our appeal to you: "I appeal to you, brethren, by the name of our Lord Jesus Christ, that all of you agree and that there be no dissensions among you, but that you be united in the same mind and the same judgment."

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And now as We come to the end of this encyclical letter, We turn Our mind to you, reverently and lovingly, beloved and venerable brothers in the episcopate, with whom We share more closely the care of the spiritual good of the People of God. For We invite all of you, We implore you, to give a lead to your priests who assist you in the sacred ministry, and to the faithful of your dioceses, and to devote yourselves with all zeal and without delay to safeguarding the holiness of marriage, in order to guide married life to its full human and Christian perfection. Consider this mission as one of your most urgent responsibilities at the present time. As you well know, it calls for concerted pastoral action in every field of human diligence, economic, cultural and social. If simultaneous progress is made in these various fields, then the intimate life of parents and children in the family will be rendered not only more tolerable, but easier and more joyful. And life together in human society will be enriched with fraternal charity and made more stable with true peace when God's design which He conceived for the world is faithfully followed.

exerpted from Humanae Vitae, written by Pope Paul VI (emphases mine)