If marriage existed long before the world's major religions...?

Why do we have to accept the doctrine that marriage MUST (in big capital bold letters) be between a man and a woman, and not between a man and a man, or a woman and a woman? Shouldn't marriage be defined by whoever decides to partake in whatever marriage is? And furthermore why does everybody say it shouldn't happen just because the bible/torah/koran says so? Please keep in mind when answering this question that in the UNited States, we "supposedly" have a separation of church and state.