G-d defined marriage.
Governments can call a cat a mouse if they wish.
Not to dispute this, we may say that God defines marriage not only through pronouncements but in nature, and I find evidence for this, in particular, in the outrage that the Roman satirist Juvenal felt and expressed in his description of "gay marriage" as he saw it being practiced in decadent Roman society circa 100 A.D. He presciently noted, "Yes, and if we only live long enough, we shall see these things done openly: people will wish to see them reported among the news of the day." I feel sure he would have been amazed that this would come true at an interval of 2000 years hence, although he did say as well, "To these ways of ours Posterity will have nothing to add."