The definition of marriage thing

I keep hearing from people against gay marriage that "the Bible says" that marriage is defined as between one man and one woman, yet I have yet to see that definition in any Bible. However, I've been reading a book about England in the twelfth century. During that time, the definition of marriage became codified. Secular law decided that canon law would decide, and canon law decided that the primary definition was that two people agree and say publicly that they were married. And at this time canon law defined marriage as an agreement between two people (which at the time would be one man and one woman). Aha! I bet that's where all this is coming from.