Because of society, which is modeled after human nature, recoils at the thought of anything unnatural and seeks to isolate itself from the offending party.
Christianity states that we should love one another, be kind, tolerant, forgiving, and help those who face difficulty and differences, especially physical ones.
In the case of homosexuality, setting aside the question of sin, there is a physical difference behind the difference in behaviors, much like the corporal difference in a leper or a blind man. Christ sought out the faithful with disabilities, and made them whole. Christianity would do the same, rather than cast them out, as if they were lepers.
It's not Christianity's fault - it's society's fault. It's human nature to abhor that which you are not, especially if the threat of a similar fate faces you. Christians can't accept is any more than any other group can accept it. Don't blame the religious right for creating a problem that really resulted from a biological difference in the brain.