This article discusses the religious freedom aspect of same-sex marriage. Way down on page two I find this gem:
"According to a 2008 survey taken by Public Religion Research, when asked whether they would favor allowing gay couples to marry "if the law guaranteed that no church or congregation would be required to perform marriages for gay couples," support for legalizing same-sex marriage jumped from 29 to 43 percent."
Good then, write it into the law. Never mind that churches already have such a right guaranteed by our constitutions, and never has a church been required to santify a marriage against its will.
In fact my church (RCC) would not sanctify my marriage just because my wife is Protestant.
Let it be so though, write it into the law. In fact let's pass a new law. Hell, make it a Constitutional Amendment.
I would word it thusly:
"In keeping with the teachings of our Lord and Savior, Jesus Christ, Christians shall reserve the right to refuse to serve or to associate with anyone who does not meet their standards.