As such, they bear witness to the possibilities of the art of medicine. But they must be given a moral evaluation in reference to the dignity of the human person, who is called to realize his vocation from God to the gift of love and the gift of life. If, on the other hand, the procedure were to replace the conjugal act, it is morally illicit. The Church approves of what is artificial when it facilitates nature and disapproves of it when it violates nature. Acts that violate the rational free, and relational nature of the human being are thereby immoral.
Contraception violates the dignity of the human person in several ways.
It reduces the sexual act to a merely biological act on the level of animal existence. Humans freely and rightly contracept and sterilize other animals because their existence and their sexuality have a merely instrumental value. Their sexuality is ordained to the propagation of the species, and since species are ultimately ordained to the benefit of human beings, we can manipulate them to our benefit. The human sexual act is not manipulable in the same way since human life is of intrinsic value. There are strict conditions for the moral performance of the human sexual act, conditions mandated by the vulnerability of the human person and the value of human life.