50 Marriage and conjugal love are by their nature ordained toward the begetting and educating of children. Children are really the supreme gift of marriage and contribute very substantially to the welfare of their parents. The God Himself Who said, "it is not good for man to be alone" (Gen. 2:18) and "Who made man from the beginning male and female" (Matt. 19:4), wishing to share with man a certain special participation in His own creative work, blessed male and female, saying: "Increase and multiply" (Gen. 1:28). Hence, while not making the other purposes of matrimony of less account, the true practice of conjugal love, and the whole meaning of the family life which results from it, have this aim: that the couple be ready with stout hearts to cooperate with the love of the Creator and the Savior. Who through them will enlarge and enrich His own family day by day. Parents should regard as their proper mission the task of transmitting human life and educating those to whom it has been transmitted. They should realize that they are thereby cooperators with the love of God the Creator, and are, so to speak, the interpreters of that love. Thus they will fulfil their task with human and Christian responsibility, and, with docile reverence toward God, will make decisions by common counsel and effort. Let them thoughtfully take into account both their own welfare and that of their children, those already born and those which the future may bring. For this accounting they need to reckon with both the material and the spiritual conditions of the times as well as of their state in life. Finally, they should consult the interests of the family group, of temporal society, and of the Church herself. The parents themselves and no one else should ultimately make this judgment in the sight of God. But in their manner of acting, spouses should be aware that they cannot proceed arbitrarily, but must always be governed according to a conscience dutifully conformed to the divine law itself, and should be submissive toward the Church's teaching office, which authentically interprets that law in the light of the Gospel. That divine law reveals and protects the integral meaning of conjugal love, and impels it toward a truly human fulfillment. Thus, trusting in divine Providence and refining the spirit of sacrifice, married Christians glorify the Creator and strive toward fulfillment in Christ when with a generous human and Christian sense of responsibility they acquit themselves of the duty to procreate. Among the couples who fulfil their God-given task in this way, those merit special mention who with a gallant heart and with wise and common deliberation, undertake to bring up suitably even a relatively large family. Marriage to be sure is not instituted solely for procreation; rather, its very nature as an unbreakable compact between persons, and the welfare of the children, both demand that the mutual love of the spouses be embodied in a rightly ordered manner, that it grow and ripen. Therefore, marriage persists as a whole manner and communion of life, and maintains its value and indissolubility, even when despite the often intense desire of the couple, offspring are lacking.
Source: Gaudium et Spes (Vatican.va)