Family planning is a responsibility of marriage. “A particular aspect of this responsibility concerns the regulation of procreation. For just reasons, spouses may wish to space the births of their children.” (CCC 2368) But this should not be accomplished through immoral means. Contraception is defined as: “The use of mechanical, chemical, or medical procedures to prevent conception from taking place as a result of sexual intercourse; contraception offends against the openness to procreation required of marriage and also the inner truth of conjugal love.” (CCC glossary)
The rhythm method and natural family planning (NFP) are not forms of contraception but rather methods of responsibly regulating births. They do not “prevent conception from taking place as a result of sexual intercourse” thereby closing the act to the gift of life.
The catechism explains: “Periodic continence, that is, the methods of birth regulation based on self-observation and the use of infertile periods, is in conformity with the objective criteria of morality. These methods respect the bodies of the spouses, encourage tenderness between them, and favor the education of an authentic freedom. In contrast, “every action which, whether in anticipation of the conjugal act, or in its accomplishment, or in the development of its natural consequences, proposes, whether as an end or as a means, to render procreation impossible” is intrinsically evil: Thus the innate LANGUAGE that expresses the total reciprocal self-giving of husband and wife is overlaid, through contraception, by an objectively contradictory LANGUAGE, namely, that of not giving oneself totally to the other. This leads not only to a positive refusal to be open to life but also to a falsification of the inner truth of conjugal love, which is called upon to give itself in personal totality. . . . The difference, both anthropological and moral, between contraception and recourse to the rhythm of the cycle . . . involves in the final analysis two irreconcilable concepts of the human person and of human sexuality.” (CCC 2370)