9 In the light of these facts the characteristic features and exigencies of married love are clearly indicated, and it is of the highest importance to evaluate them exactly. This love is above all fully human, a compound of sense and spirit. It is not, then, merely a question of natural instinct or emotional drive. It is also, and above all, an act of the free will, whose trust is such that it is meant not only to survive the joys and sorrows of daily life, but also to grow, so that husband and wife become in a way one heart and one soul, and together attain their human fulfillment. It is a love which is total—that very special form of personal friendship in which husband and wife generously share everything, allowing no unreasonable exceptions and not thinking solely of their own convenience. Whoever really loves his partner loves not only for what he receives, but loves that partner for the partner's own sake, content to be able to enrich the other with the gift of himself. Married love is also faithful and exclusive of all other, and this until death. This is how husband and wife understood it on the day on which, fully aware of what they were doing, they freely vowed themselves to one another in marriage. Though this fidelity of husband and wife sometimes presents difficulties, no one has the right to assert that it is impossible; it is, on the contrary, always honorable and meritorious. The example of countless married couples proves not only that fidelity is in accord with the nature of marriage, but also that it is the source of profound and enduring happiness. Finally, this love is fecund. It is not confined wholly to the loving interchange of husband and wife; it also contrives to go beyond this to bring new life into being. "Marriage and conjugal love are by their nature ordained toward the procreation and education of children. Children are really the supreme gift of marriage and contribute in the highest degree to their parents' welfare." (8)
Source: Humanae Vitae (Vatican.va)