In recent discussion of same-sex marriage, I've been pleased to see a particular term rising to new prominence: "civil marriage."
I think use of that term neatly encapsulates and clarifies a distinction that some opponents of same-sex marriage have been working to muddy: the distinction between marriage sanctioned by the government and religious marriage.
Nearly all discussions of same-sex marriage laws are, of course, talking exclusively about civil marriage. But those who argue against those laws are most often talking about religious marriage. And it's only been recently that I've seen same-sex marriage advocates start to regularly use the term "civil marriage" to make that distinction explicit. (Though it's entirely possible that people have been using that term for ages and I just haven't noticed.)
For example, Maine's LD 1020 (the same-sex marriage bill signed into law the other day) is titled "An Act To End Discrimination in Civil Marriage and Affirm Religious Freedom."
I was reminded to mention this by an opinion piece from Bloomberg yesterday, which provides a useful analogy in passing: "The state can't force an Orthodox rabbi to marry Jew to non-Jew."
So maybe that's one way to reframe this argument:
Oh no! If non-Christian opposite-sex civil marriage were legal, then the state would force Christian churches to perform weddings of non-Christians! An atheist man and an atheist woman could force a church to perform a marriage ceremony for them, with all the power of the state behind them! Ministers and priests would be required by the government to perform marriage ceremonies that went against their consciences!
. . . Wait, what's that you say? Opposite-sex marriage for non-Christians is legal? And the state doesn't force Christian churches to perform any particular sort of weddings?
Note: it's important to remember that plenty of religious people, including plenty of Christians, are actually in favor of same-sex marriage, of the civil and/or religious varieties. My argument/analogy above isn't intended to suggest otherwise; it's addressed only to religious people (of any faith) who are concerned that legalizing same-sex civil marriage would force those churches that oppose same-sex religious marriage to perform such weddings.