Whenever I hear talk of the "sanctity" of the "sacred institution" of marriage, I think: render unto Caesar (Matt 22:21).
If marriage is blessed by God, then why should the government be in the marriage business? Let the religious take marriage to their churches, synagogues, mosques, temples and sweat lodges -- or beaches and rented halls for the secular. And let the state provide civil unions to everybody, gay or straight.
If conservatives can't accept the Solomonic idea of splitting the baby (1 Kings 3:16-27), then perhaps they'll act like the virtuous mother and offer it up to their opponents.