Well, from what I understand it's pretty complicated, and a lot of the suspicion and anger goes back to the beginning of the AIDS epidemic, when bisexual men were being blamed for going out, having unprotected sex with male prostitutes, and then coming home and infecting their wives. I remember this myth from the 90s, too.

And for gays and lesbians who still have a hatred of straight culture and its absolute domination of media (i.e., "universal" stories which are actually heterosexual stories), the distaste for bisexuality makes sense.