Why isn't gay marriage allowed? (Part 2)?

Okay, the answers from my last question were predictable. I mentioned that our Constitution holds no religion above the other. So please rule out anything having to do with God from your answers. I want your own honest opinion on why gays can't marry. I don't care what God thinks - I'm an atheist. I just want to know why two people who love each other and happen to have the same can't have the same rights of heterouals. phobia and religious bigotry aside, please offer me an answer.