I'll be honest. I am surprised to see so many conservatives who are for "gay marriage". In my opinion, conservatives are generally against it.
In my opinion, Conservatives should be defenders of Western culture and defenders the status quo. The modern status quo in Western culture is based on the enlightenment values which include among them
classical liberalism.
With regards to the US, Classical liberalism was the political philosophy of the Founding Fathers. It permeates the Constitution, the Federalist Papers and many other documents produced by the people who created the American system of government. Many emancipationists who opposed slavery were essentially classical liberals, as were the suffragettes, who fought for equal rights for women.
That said, there is no sound legal principle on which we should oppose marriage equality, and the moral ones are questionable at best.