What is a Christian's justification for a legal prohibition of homosexual marriage?

Today President Obama came out as the first U.S. president to support gay marriage. In his address, he even quoted scripture to support his change of view. Love him or hate him, it seems clear that he has struggled personally over the issue, which leads me to think that Obama probably personally opposes homosexual behavior, but has come to the conclusion that permitting homosexual marriage is the right thing, and even the Christian thing to do. This question is not about Obama's motives or his personal feelings. I simply bring this up as context, and to help explain the same intellectual struggle I'm dealing with personally. I suspect many Christians find themselves in a similar situation of opposing homosexual behavior, but feeling caught in the debate over whether their moral/personal objection to homosexuality ought to have the force of law. So for this question, I am seeking answers from the view point that homosexual behavior is a sin (there are [many](https://christianity.stackexchange.com/questions/121/what-does-the-bible-say-about-homosexuality) [other](https://christianity.stackexchange.com/questions/381/why-do-some-christians-believe-it-is-moral-to-be-a-homosexual) [questions](https://christianity.stackexchange.com/questions/1086/does-god-create-some-people-heterosexual-and-others-homosexual) that deal with this issue already, so please don't debate this issue here). My question, then, is: *Given that homosexual behavior is considered a sin, what is a Christian's justification for the legal prohibition against homosexual marriage?* I'm hoping for answers that cite Biblical principles, or at minimum theological and doctrinal principles. I'm not interested in "slippery slope" arguments. Strictly "social" justifications ("I don't want my kids growing up in that sort of society," "Legal marriage benefits exist because traditional marriage helps society in ways that homosexual marriage does not," etc), may be valid reasons to have such a view, but I don't consider them *Christian justification*, so I would ask that such answers be omitted here (unless they can some how be tied specifically into Christian doctrine). I'm also curious what, if anything, makes the homosexual marriage issue distinct among other moral issues that are legal, but not condoned by Christian teaching. Adultery, sex outside of marriage, pornography, strip clubs, gambling, smoking, etc. Should a Christian oppose the legality of all of these things as well?