There is not one mention of marriage in the US constitution.
This is true but irrelevant as it establishes principles which the legality of homosexual marriage is determined.Interpreting the Constitution requires understanding the moral framework of its writers. And which reflects a basic Christian moral ideology, which, along with every known religion then, defines marriage as btwn opposite genders, and does not confuse civil rights with grossly immoral actions.
This ideology easily gives support for abolition, (1Cor. 7:21; Philemon) but not moral abomination.
Some argue that since the Constitution does not forbid gay marriage, or restricts it to straights, then it is unconstitution to outlaw it, but which rational would also sanctiom marriage btwn man and beast, or trees, etc.,and supposes the Founders subscribed to a modern immoral ideology, contrary to the evidence.
George Washington's response to a man who had convicted for attempting to commit sodomy was not to advise him to marry, or even to institute DOMA, butr to drum him out.
Now of course, even Googling "George Washington's response homosexual relations military" (w/out quotes) results in almost half the results alleging he was gay friendly.