Let's just get this crap out of the way. It's because people have been reading the bible for centuries, interpreting it to be against homosexuality, and just about every other form of sexuality, without reason. That's the end of it.
If you believe it, question it. If you question it, get an answer. If you have an answer, does that answer satisfy reality? Does it satisfy you? Probably not. For no one else will agree with you, not really.