i find that anyone who identifies with any group is often turned off by members of any other group.
Look at any large cafeteria-- the blacks sit with the blacks, the christians certainly don't sit with the muslims, the gays sit with the gays, the goths sit with the goths and the whites sit with the whites.
And no one sits with the japanese kids.
And before one of the self-proclaimed enlightened pipe up about how they are genderbending and crossing the barriers of race and religion to embrace everyone, this is a larger scale observation of the aggregate of society, not a picture of an individual's mind.
I found this to be true in highschool, college, and the work place, but at least not graduate school, possibly due to the smaller number of total people.
I would suspect the bisexuals have this "problem" as there are not enough of them in general to create their own support network. However, I have never noticed wonton discrimination against bisexuals by gays, so I'm also going to suggest that seeing "a gay" say something nasty to "a bi" does not suggest systemic discrimination.