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 other questions 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?
From Romans 1, we understand that homosexual activity is a result of a people who know God, but choose not to glorify Him as God. In other words, it is a symptom of a society that has rejected God. 

From a theological point of view, a Christian could oppose gay marriage on the grounds that his government is, acting on his behalf, calling "evil good and good evil," directly rejecting the prescribed order. It is tantamount to the society as a whole rejecting God. As one who has a vested interest in not rejecting God, I do not want my government, acting on my behalf, endorsing that which I detest. 

This is an active endorsement of a sin, as opposed to mere toleration as would be the case with merely allowing activities that are otherwise sinful.  Indeed, all of what is listed could be considered adiaphora - it is clearly not good stuff, and is actively discouraged amongst those who choose to follow "The Way," but there is no justification for legal compulsion to the contrary.  In contrast, a government explicitly deriving its power from the consent of the governed is making a explicit endorsement of the normative nature of same-sex relationships in declaring marriage "equal" to heterosexual relationships.

(For the non Christians, a quick disclaimer. I understand we live in a pluralistic society. I realize I would be "imposing my beliefs" on others with this argument. I am simply answering the question, not even taking a stand - though today's news did greatly sadden me. Please vote on the theological justification, do not impose your "tolerance" and pluralism on me. You will note, I am not calling for the pitchforks. I'm just choosing to exercise my rights as a citizen to voice my disapproval, and to show that contrary to what people say, it does affect me.  )

Additionally, a Christian would argue that marriage is an institution ordained by God, not a function of the state. When Jesus said, "for this reason a man shall leave his mother and cling to his wife," it is because that is the order which God prescribed. A Christian response would ask why the state gets to (re-)define that which God has defined and put together. 

Fundamentally, the real question has less to do with homosexuality itself, and far more to do with the question of whether marriage is a secular or a theological institution. As one who sees marriage as predating governments, I tend to think its none of the government's business.