Basically, the nature of marriage is not a religious question. Marriage comes to us from nature. Christ sanctifies marriage as a sacrament for the baptized, giving it significance beyond its natural reality; the State protects marriage because it is essential to family and to the common good of society. But neither Church nor State invented marriage, and neither can change its nature.
Nature and Nature’s God, to use the expression in the Declaration of Independence of our country, give the human species two mutually complementary sexes, able to transmit life through what the law has hitherto recognized as a marital union. Consummated sexual relations between a man and a woman are ideally based on mutual love and must always be based on mutual consent, if they are genuinely human actions. But no matter how strong a friendship or deep a love between persons of the same sex might be, it is physically impossible for two men, or two women, to consummate a marital union. Even in civil law, non-consummation of a marriage is reason for annulment.