Bi now, gay later.
Part of the reason gays don't trust bisexuals, I think, is that, in the first place, bisexuality is seen as a sort of middle ground between heterosexuality and homosexuality; generally, it's probably more palatable to parents and to homophobic friends--if a dude at least like girls, then he isn't a total homo, the thinking goes. (I just noticed you touched on this in your original post, but I'm not going to delete all that.) There's also a bit of evidence to suggest that, at least in men, it doesn't really exist; most men who identify as bisexual are almost exclusively into men or exclusively into women. Also, it's trendy. In a poll on another forum I just happened to be reading today, just under half the population went both ways, apparently. It wasn't an LGBT forum or anything.
Anyway, I haven't noticed a lot of discrimination against bisexuals in general, but maybe that's because I'm not bisexual. "Pansexuality," however, seems to be the cool thing now at my school's GSA and among the supportive people I know.