I'm a lesbian and I don't hate bisexuals. I don't know any gays who hates them, and I work in a LGBT organisation (and app. 70% of the employees are some variation of LBGT).
But maybe I can offer an explanation:
1) Lesbians get flooded with online offers from bi women who are A) in a relationship with a McDude but wants to follow her urges (with dudebros acceptance), or B) wants us to join them in a threesome.
This leaves us with 2 impressions:
In the first case what I'm internalizing is that at some point I might have to accept that my SO will want to sleep with men (aka other people), and I'm not cool with that, since I'm horribly monogamous. Dudebro is simply a better person than me, or he's poly (or theres a 3. reason).
In the case of the threesome, I feel disrespected. Why keep asking me for a threesome, when you know I'm not gonna have sex with your dude? Many of these offers come from straight couples, and let me just quickly go ahead and give a big "FUCK YOU" to those couples. You being ok with sleeping with me, IS NOT the important factor here. Get another couple. Hey, or maybe someone bi?
Unfortunately the bi women who are looking for a monogamous relationship with a woman, kinda just blends in and it's an uphill battle that I'm glad I don't have to deal with.
2) Many gay people go through the bi phase. And I have a feeling that the next phase is called "Oh, I'm super gay, I guess bis don't exist".
I went through that phase (helped by a lot of people telling me "it's just a phase"), and then I went through the "bis don't exist" phase.
My point is that I think those phases might have a lot to do with age. I think this kind of mainly happens in the early 20's.
But an American lesbian told me that things are a lot worse in the LBGT community in America (almost no contact between gays and lesbians), so maybe it could be a american/cultural thing?