This article is from the BiSexuality FAQ, by Jon Harley email@example.com with numerous contributions by others.
One reason is because we are sometimes perceived as "hiding," a sense that
some bisexuals use their bisexuality to look heterosexual at work, in
straight social settings, to enjoy the "heterosexual privilege" that is part
of the social norm. Secondly, bisexuals are sometimes seen as blurring the
issues and weakening the lesbian and gay movement. Naturally, bisexual
activists disagree with this view (we feel that the real issue is sexual
freedom for all sexualities), but sometimes lesbians and gays label bisexuals
"traitors" for this reason. A further reason is that some lesbians and gay men
also have sex with MOTOS (while not identifying as bisexual). Often peer
pressure means that they can't admit this in the lesbian and gay communities,
and see bisexuality as a threat to their own acceptance. And finally, simply
because of the fear that arises out of ignorance and out of the media's very
poor record of portraying bisexuals as serial killers, homophobes and
generally self-centred, confused people.
The lesbian and gay communities are oppressed by homophobia and prejudice, but
unfortunately being oppressed is no guarantee that you won't oppress others.
Happily, prejudice against bisexuals in the lesbian and gay communities seems
to be diminishing over time as more people come to accept that sexuality is
not a monochrome issue.