Catechism of the Catholic Church
a, c, d, and e. Para. 1644: The love of the spouses requires, of it very nature, the unity and indissolubility of the spouses' community of person, which embraces their entire life: "so they are no longer two, but one flesh" (Mt 19:6; cf. Gen 2:24). . . .
Para. 1646: By its very nature conjugal love requires the inviolable fidelity of the spouses. This is the consequence of the gift of themselves which they make to each other. Love seek to be definitive; it cannot be an arrangement "until further notice." The "intimate union of marriage, as a mutual giving of two persons, and the good of the children, demand total fidelity from the spouses and require an unbreakable union between them" (GS 48 § 1).
Para. 1652: "By its very nature the institution of marriage and married love is ordered to the procreation and education of the offspring and it is in them that it finds its crowning glory" (GS 48 § 1; 50).
Children are the supreme gift of marriage and contribute greatly to the good of the parents themselves. God himself said: "It is not good that man should be alone," and "from the beginning [he] made them male and female"; wishing to associate them in a special way in his own creative work, God blessed man and woman with the words: "Be fruitful and multiply." Hence, true married love and the whole structure of family life which results from it, without diminishment of the other ends of marriage, are directed to disposing the spouses to cooperate valiantly with the love of the Creator and Savior, who through them will increase and enrich his family from day to day (GS 50 § 1; cf. Gen 2:18; Mt 19:4; Gen 1:28).
Para. 2366: Fecundity is a gift, an end of marriage, for conjugal love naturally tends to be fruitful. A child does not come from outside as something added on to the mutual love of the spouses, but springs from the very heart of that mutual giving, as its fruit and fulfillment. So the Church, which "is on the side of life" (FC 30) teaches that "it is necessary that each and every marriage act remain ordained per se to the procreation of human life" (HV 11). This particular doctrine, expounded on numerous occasions by the Magisterium, 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" (HV 12; cf. Pius XI, encyclical, Casti connubii).
Para. 1654: Spouses to whom God has not granted children can nevertheless have a conjugal life full of meaning, in both human and Christian terms. Their marriage can radiate a fruitfulness of charity, of hospitality, and of sacrifice.