C. S. Lewis wrote:
“Before leaving the question of divorce, I should like to distinguish two things which are very often confused. The Christian conception of marriage is one: the other is the quite different question-how far Christians, if they are voters or Members of Parliament, ought to try to force their views of marriage on the rest of the community by embodying them in the divorce laws. A great many people seem to think that if you are a Christian yourself you should try to make divorce difficult for every one. I do not think that. At least I know I should be very angry if the Mahommedans tried to prevent the rest of us from drinking wine. My own view is that the Churches should frankly recognise that the majority of the British people are not Christians and, therefore, cannot be expected to live Christian lives. There ought to be two distinct kinds of marriage: one governed by the State with rules enforced on all citizens, the other governed by the Church with rules enforced by her on her own members. The distinction ought to be quite sharp, so that a man knows which couples are married in a Christian sense and which are not.”
This passage suggests that we should operate with two distinct conceptions of marriage, and that we should not be looking to the government to uphold the Christian conception. While this does not provide a case for same sex marriage, it does undercut certain ways of arguing against it. Some people argue that there is a univocal conception of marriage ordained by God when he created Adam and Eve (not Adam and Steve), and that that conception must be upheld by government. A person can start an affair, divorce his wife, and then marry the person he is having an affair with, and if he does so, the government asks only if the divorce was executed legally. Despite being the "guilty party" in the collapse of his prior marriage, he is free to marry again. If we go with Lewis on this, government is free to decide who is married and who is not as it sees fit for its own purposes, and this may or may not fit with what Christian church might believe. So, opponents of gay marriage need to find another line of argument.
I've always considered government to be a poor guardian of the sacred institution of marriage. I would expect that people who think government should be limited would recognize the limitations of government in this.
Here is a blog post to the same effect.