OK, I'm probably going to get lambasted here, but here we go anyway.
I fully understand why many "100% gay" people have a problem with bisexuals in general. Now this is largely based upon my experience with the GLBT community, and as such is completely biased to my experience, but by and large most of the bisexuals I've met are only bisexual when it comes to sex and straight when they are introducing their partner to their families.
I think we see far too many "weekend gays", especially because being bisexual is all the rage right now.
Many feel that it minimizes the struggle that gay men and women have gone through in accepting their sexuality, and being accepted by others.
Personally, I could care less what your sexuality is, but I can see where gay men and women, especially ones from generations who were forced to live a lie most of their lives, would feel slighted by bisexuals hitching their wagon onto the gay rights movement.
As to transgendered folk, I can't even begin to see why they're thrown to the wolves by the gay community as they are/were just as stigmatized, if not more so than gays and lesbians.
All that being said, at the end of the day, we're all human and don't like what's different and that happens in all groups.
I'll close with a line from Torch Song Trilogy. "Just once, I'd like to meet a bisexual who brings his boyfriend home to his mother and steps out with his woman on the weekends."