In American law, certain conditions of marriage have always been nonnegotiable. A marriage joins (a) two people (b) of the opposite sex (c) who are not close relatives. Under that venerable definition, there can be no valid same-sex marriage, no polygamous or other plural marriage, and no incestuous marriage. But if the opposite-sex requirement is an unconstitutional infringement on the right to marry - which the California court explains as "the right of an individual to establish a legally recognized family with the person of one's choice" - then so are the restriction of marriage to two people and the ban on incestuous marriage. If two women who wish to marry each other must be permitted to do so, why not two sisters? Why not three?To which I would add, the modern conception of marriage is a direct outgrowth of Christian theology. It owes little debt to Celtic pagan, Greco-Roman, or Near Eastern marriage rites and traditions. Those were all contractual; the covenantal and equally binding nature of western marriage is a direct product of Christianity. Moreover, marriage was originally the province of the church - the state decided to join the church in its marital mission and created civil marriage on the church model. Current judicial changes essentially amount to a hostile takeover.
...Men and women are not interchangeable, and same-sex unions - no matter how devoted and enduring - cannot take the place of a married husband and wife. The essential function of marriage is to unite male and female. That is the only kind of union that can produce new life, and therefore the only kind of union in which society has a survival stake.
The government may rightly abandon marriage, admitting its lack of competence to oversee an essentially cultural and religious matter. It may agree to be the arbiter of contracts between persons of any nature - with no restrictions whatsoever, but also no pretense to marriage. Or it may judge that marriage is socially valuable, and continue to aid the church in enforcing and promulgating the institution. But the government did not define marriage and has no authority to redefine it.