Two reasons: first off, there's a silent majority of Christians who may believe that homosexual acts are wrong but also recognize the fact that there's no such thing as a perfect person.

Secondly: in any given group of people, there are bound to be ignorant jerks who justify their own viewpoint by over-simplifying an opposing group's stance. *(see: bigoted Christian pastors who ostracize gays.)

Point being, when you think about it, it's actually two sides of the same coin. Each group has its crazies, but most people are pretty decent.