From Pope St. John Paul II’s Apostolic Exhortation Familiaris Consortio (1981):
According to the plan of God, marriage is the foundation of the wider community of the family, since the very institution of marriage and conjugal love are ordained to the procreation and education of children, in whom they find their crowning.34
In its most profound reality, love is essentially a gift; and conjugal love, while leading the spouses to the reciprocal “knowledge” which makes them “one flesh,”35 does not end with the couple, because it makes them capable of the greatest possible gift, the gift by which they become cooperators with God for giving life to a new human person. Thus the couple, while giving themselves to one another, give not just themselves but also the reality of children, who are a living reflection of their love, a permanent sign of conjugal unity and a living and inseparable synthesis of their being a father and a mother.
When they become parents, spouses receive from God the gift of a new responsibility. Their parental love is called to become for the children the visible sign of the very love of God, “from whom every family in heaven and on earth is named.”36
It must not be forgotten however that, even when procreation is not possible, conjugal life does not for this reason lose its value. Physical sterility in fact can be for spouses the occasion for other important services to the life of the human person, for example, adoption, various forms of educational work, and assistance to other families and to poor or handicapped children.
34 Cf. Pastoral Constitution on the Church in the Modern World Gaudium et spes, 50.
35 Cf. Gn 2:24.
36 Eph 3:15.
— Familiaris Consortio 14