"Marriage" in the sense of the word used in the current controversy, is strictly a legal contract. The religious notion of "marriage" is whatever each religion decides. In this sense, gays have been married for decades already. No religion is allowed to own the concept or define it's prerequisites.
All we're talking about is the secular legal and financial contract of marriage, which has NO religious component. I've been to several marriages where the word "God" was never used, and they are legal, binding marriages.