21 The right and lawful ordering of birth demands, first of all, that spouses fully recognize and value the true blessings of family life and that they acquire complete mastery over themselves and their emotions. For if with the aid of reason and of free will they are to control their natural drives, there can be no doubt at all of the need for self-denial. Only then will the expression of love, essential to married life, conform to right order. This is especially clear in the practice of periodic continence. Self-discipline of this kind is a shining witness to the chastity of husband and wife and, far from being a hindrance to their love of one another, transforms it by giving it a more truly human character. And if this self-discipline does demand that they persevere in their purpose and efforts, it has at the same time the salutary effect of enabling husband and wife to develop to their personalities and to be enriched with spiritual blessings. For it brings to family life abundant fruits of tranquility and peace. It helps in solving difficulties of other kinds. It fosters in husband and wife thoughtfulness and loving consideration for one another. It helps them to repel inordinate self-love, which is the opposite of charity. It arouses in them a consciousness of their responsibilities. And finally, it confers upon parents a deeper and more effective influence in the education of their children. As their children grow up, they develop a right sense of values and achieve a serene and harmonious use of their mental and physical powers.
Source: Humanae Vitae (Vatican.va)