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. The sexual relationship between spouses is one of God s great gifts to mankind. Pope John Paul II teaches that it enables human beings to overcome their existential loneliness and to overcome their selfishness by participating in a common good and a good of inestimable value the good of bringing into existence another creature with an immortal soul. The sexual act is morally performed only within marriage where it has the ordination both to deepening the union of the spouses and to building a family.
Contraception violates both meanings of the conjugal act.
The conjugal act is meant to be an act of total self-giving, which includes giving the power of becoming a parent with another. They are not achieving the union proper to spouses, a union that respects the baby-making power of the sexual act as a proper fulfillment of spousal union. Contraception treats the procreative meaning of the sexual act as though it were an impediment to spousal union. Thus, contraception negates the good of the openness to a new immortal life that is inherent in the sexual act of humans and trivializes the commitment that is implied in the conjugal act. On the other hand, couples using methods of natural family planning are respecting the nature of the conjugal act through their acts of self-denial.