51 This council realizes that certain modern conditions often keep couples from arranging their married lives harmoniously, and that they find themselves in circumstances where at least temporarily the size of their families should not be increased. As a result, the faithful exercise of love and the full intimacy of their lives is hard to maintain. But where the intimacy of married life is broken off, its faithfulness can sometimes be imperiled and its quality of fruitfulness ruined, for then the upbringing of the children and the courage to accept new ones are both endangered. To these problems there are those who presume to offer dishonorable solutions indeed; they do not recoil even from the taking of life. But the Church issues the reminder that a true contradiction cannot exist between the divine laws pertaining to the transmission of life and those pertaining to authentic conjugal love. For God, the Lord of life, has conferred on men the surpassing ministry of safeguarding life in a manner which is worthy of man. Therefore from the moment of its conception life must be guarded with the greatest care while abortion and infanticide are unspeakable crimes. The sexual characteristics of man and the human faculty of reproduction wonderfully exceed the dispositions of lower forms of life. Hence the acts themselves which are proper to conjugal love and which are exercised in accord with genuine human dignity must be honored with great reverence. Hence when there is question of harmonizing conjugal love with the responsible transmission of life, the moral aspects of any procedure does not depend solely on sincere intentions or on an evaluation of motives, but must be determined by objective standards. These, based on the nature of the human person and his acts, preserve the full sense of mutual self-giving and human procreation in the context of true love. Such a goal cannot be achieved unless the virtue of conjugal chastity is sincerely practiced. Relying on these principles, sons of the Church may not undertake methods of birth control which are found blameworthy by the teaching authority of the Church in its unfolding of the divine law. All should be persuaded that human life and the task of transmitting it are not realities bound up with this world alone. Hence they cannot be measured or perceived only in terms of it, but always have a bearing on the eternal destiny of men.
Source: Gaudium et Spes (Vatican.va)