Why Can Marriage Exist Only Between a Man and a Woman?

The natural structure of human sexuality makes man and woman complementary partners for the transmission of human life. Only a union of male and female can express the sexual complementarity willed by God for marriage. The permanent and exclusive commitment of marriage is the necessary context for the expression of sexual love intended by God both to serve the transmission of human life and to build up the bond between husband and wife (see CCC, nos. 1639-1640).

In marriage, husband and wife give themselves totally to each other in their masculinity and femininity (see CCC, no. 1643). They are equal as human beings but different as man and woman, fulfilling each other through this natural difference. This unique complementarity makes possible the conjugal bond that is the core of marriage.

Why is a same-sex union not equivalent to a marriage?

For several reasons a same-sex union contradicts the nature of marriage: It is not based on the natural complementarity of male and female; it cannot cooperate with God to create new life; and the natural purpose of sexual union cannot be achieved by a same-sex union. Persons in same-sex unions cannot enter into a true conjugal union. Therefore, it is wrong to equate their relationship to a marriage.