Uh, how about 1791?

"I'm curious, when did it become unconstitutional to exclude homosexual couples from marriage?"

I don't get too wound up about 'marriage equality'. I wouldn't walk across the street to vote for it, or against it. I don't care. But, I do know one thing, 'marriage' isn't mentioned even once in the Constitution, but equal protection under the law, is. Seems pretty simple to me.