And my response is invariable No, this is not a Christian issue.  This is a human issue.  If you want to make a religious issue out of it, then you have to deal with the fact that no major religion argues for gay marriage or even comes close to anything equating the two.  Biblical Christianity explicitly forbids homosexuality in general, let alone same sex marriage.  Islam is even less forgiving.  Judaism is historically very strong against it.  Buddhism, Hinduism - none of them endorse homosexuality, though to be fair they may be silent on the issue.  And, as clarification, there are splinter groups of Christianity and Judaism (and perhaps other religions as well) that have come out in favor of homosexuality and/or homosexual marriage - but these are almost entirely products of the 20th century and are hardly representative of the best historical orthodox positions of their respective religious traditions.  But the main point is that if you want to make it a religious issue (and of course, as a Biblical Christian who believes that there is a God who created us all and has very definite notions of how we ought to be, I believe everything is ultimately a religious issue), then it's inaccurate to confine the discussion to Christianity, as though Biblical Christianity is the equivalent of the slow child in the back of the classroom or the playground bully demanding that everyone play by his rules.  

Particularly as concerns the role of the State in the issue (and it has a very big, definite, historical role), I think this article does an admirable job of laying out an intelligent secular argument against redefining marriage to include homosexual relations and any number of other possible combinations.  Thanks to a new friend, Justine, for making me aware of the article.