As we have seen, however, marriage is a great deal more than a relationship of interdependence between consenting adults. It aims at much more than the well-being and fulfillment of the partners. It possesses another constituent element, namely, the procreative potential of the man and woman who are making the commitment.
The sexual relationship between two men or two women is not equivalent to the sexual relationship between a man and a woman because they do not have the biological capacity to generate new lives.
It must also be added that with regard to education of children, the same values cannot reasonably be attributed to both types of union. The principal right of children is to be born of an act of love and to live in complete communion with a father and mother.
Therefore, it is neither unjust nor discriminatory to name and treat differently two realities that are so intrinsically different both anatomical and psychoaffective perspective. On the contrary, it would be unjust and discriminatory toward married heterosexual couples to treat them this way.
The State must accord special treatment to a man and woman who marry, not because of the exclusivity, dependence, duration or sexual nature of their union, but because of its vital function of procreation and its function of socialization that encourages complementarity between man and woman for the greater good of their children. (...) Read the whole thing at the CCCB's Website.