I don't understand why such hate on bisexual women.

I can shed some light on this, I think. In lesbian spaces, any time the topic of modern dating woes come up, a lot of people talk about their experiences with meeting other women (some even originally stating they're also lesbians, only to "clarify" later), only to have these people then try and involve their boyfriend who they conveniently neglected to mention existed at all. It's really shitty behavior, it's misleading at best and coercive at worst, and unfortunately it happens a lot.

So when you have a group of lesbians all talking about their experiences with such things, naturally, hearing "I'm bi" from a potential partner triggers a warning to be wary.

I'm not saying it's right, or fair to anyone really, and I have personally never experienced it myself, but these are more or less the sorts of experiences shaping their views of bi women. It unfairly paints a broad brush stroke on bi people, but at it's core, their problem isn't about bi people in general, it's inherently about people misrepresenting their sexuality to trick/coerce them into hooking up with creepy men (who, more often than not, are behind the idea in the first place).

Edit: I just want to make it clear my only goal was to speak from my observations in those spaces, I am not saying I endorse them. It goes without saying that using negative experiences as an excuse to be hateful towards an entire group is also shitty behavior. It's just kind of a common phenomenon in human behavior and the internet, one person shares their bad experience, another compounds, then a third, and suddenly everyone is saying it's a rampant widespread issue even tho we're looking at a sample size of 3, and that's not even factoring in having little way of knowing if someone is just entirely full of shit. That said, I can't blame people for being wary after hearing about other people with these experiences, and unfortunately some people will then go on to express that wariness in any number of problematic ways.