Thank you, though I'm far from the first person to describe marriage that way (it's pretty much straight out of the Bible), many people fail to make the connection when it comes to same-sex love.
I don't want to make a blanket statement here: I speak only for myself, but I, as a progressive Christian, through a series of events unrelated to progressive politics came to realize that the generally accepted interpretation the Bible's passages generally used to tear down gay and lesbian people have been gravely and tragically misinterpreted by people looking for justification for their prejudice. As such, I believe in same-sex marriage (homosexual marriage is a loaded term, on par with "Obamacare" and other such buzzwords), for the same reason I believe in different-sex marriage: because marriage is about loving another person the way God loved the church, and that kind of love knows no gender