You're right, it's Gods, I never said it was mine to define. He defined it, the creator of you, the one who made all of us, the one who created marriage in the first place, has defined it.
But why are you ignoring J.Allen
's comments that clearly show, "God" not only "defined" marriage as "one man and one woman", but made a whole lot of other stipulations too, such as that a woman has to be a virgin before marriage and re-marrying after divorce is adultery?
I don't recall there being any movement to outlaw divorce and re-marriage, despite those things being undoubtedly a much bigger threat to straight marriage than gays opting in.