Mater et Magistra §193

¹⁹³ We must solemnly proclaim that human life is transmitted by means of the family, and the family is based upon a marriage which is one and indissoluble and, with respect to Christians, raised to the dignity of a sacrament. The transmission of human life is the result of a personal and conscious act, and, as such, is subject to the all-holy, inviolable and immutable laws of God, which no man may ignore or disobey. He is not therefore permitted to use certain ways and means which are allowable in the propagation of plant and animal life.