A common line trotted out by the homophobic crowd is, “It’s Adam and Eve, not Adam and Steve!” It’s not clear what exactly their point is, but I suppose it’s that, since the first marriage in existence was that of a man and a woman, all the others should be too. So let’s look at Adam and Eve.
God made her from his (Adam’s) rib, the two of them broke the only rule they were given because they listened to a talking snake, and their first son murdered the second. Oh, and it’s possible that she wasn’t his first wife. Sounds like a great model for every couple, right? I’m not entirely sure how this is against same-sex marriage anyway, when the woman was the only other person on Earth. Maybe Adam WAS gay, but there was no way to know when there weren’t any other men. It’s not like God made a man AND a woman out of Adam’s rib, he chose the woman, and God declared that everyone else should do that as well. Besides, doesn’t marrying someone made from your own body sound kind of incestuous?
That’s one reason why I think the idea of the entire human race being started by just two people, as common as it is, probably wasn’t taken literally even by the people who came up with it. These societies usually had taboos against incest, but if we’re all the descendants of a single couple, then the first few generations had no choice EXCEPT incest. And don’t forget that incest is one of the things that homophobes think gay marriage will lead toward. Another is polygamy, and while Adam wasn’t a polygamist (well, unless you accept the Lilith story, as divorce presumably hadn’t been invented yet), plenty of people in the Bible were. I’ve heard people try to rationalize this by saying polygamy is always presented in a negative light in the Bible, and I’ll admit that it SOMETIMES is, but most of the time it’s pretty matter-of-fact. As for bestiality, God paraded all the animals before Adam to see if he wanted to choose any of them as a helpmeet. He ultimately refused, but it sounds like God would have been okay if he’d taken that option.
Really, talking about the Biblical definition of marriage is kind of absurd, because I don’t think the Bible ever really defines marriage. It establishes rules pertaining to marriage, but the institution itself is presented as one with which readers would already be familiar. And a lot of the rules in the Bible are totally irrelevant in modern society anyway. We’ve ALREADY redefined marriage in many ways, so pointing to tradition isn’t really that helpful to your case.\