Let the Catechism of the Catholic Church answer:
2370 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.157 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:158
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."
vatican.va/archive/ENG0015/__P86.HTM
While both NFP and contraception may be licit in intention when they intend to prevent a grave evil, only NFP can remain objectively compatible with the true identity/purpose of human sexuality: that it should involve 1) “**total **mutual self giving between spouses” and it should remain 2) open to life.
In NFP, the spouses still objectively give themselves fully to one another as they are during the sexual act because there is no attempt to DECREASE one’s fertility, unlike in contraception.
In NFP, the spouses can still be objectively open to life. While they intend not to have children for grave reasons, they only desire it insofar as the the natural properties of fertility and sex allow it. In the end, they respect the fact that their intention not to have a child is to be limited by the natural properties which God endowed in sexuality. They respect the possibility that this sexual act can still bring forth life according to its own properties. In contraception, one is objectively closed to life/without respect to the procreative purpose of sexuality because there is a positive attempt to decrease one’s fertility or render procreation impossible.