A good column from townhall.com‘s Mike Adams:
A lot of cultural commentators are confused these days. They believe that people’s views on same sex marriage are solely a reflection of their religious beliefs. Nothing could be further from the truth. Actually, some things could be further from the truth – like saying that Al Sharpton has integrity or that Dan Savage has class. But you get the point. The same sex marriage debate is about politics. To call it a religious debate is to miss the point entirely. Your stance on same sex marriage should vary depending on whether you consider yourself to be a conservative, a liberal, or a libertarian.
For conservatives, the issue is pretty simple. The institution of marriage predates any existing government or nation. So no government has a right to redefine marriage. But it is okay for the government to become entangled with marriage towards the end of promoting marriage. The institution is good. It tames men. It protects women. It is good for children. Therefore, it is worth promoting.