Here's my take on it. This is mostly stuff I've said before in previous threads.
I know a lot of lesbians who have had bad experiences with girls who aren't lesbians breaking their hearts because of going back to men or cheating on them with men which has sort of soured them on non-lesbians. I really try, personally, to be open to the girls who are bi/pan/unlabeled/whate ver because, in theory, I embrace the idea that love is about a person and not a gender. I, myself, have never been turned on by any male-identified or male-bodied person and would never want to be in a relationship where the other person doesn't turn me on, so I stick with what works for me. But in THEORY, the equal opportunity mindset jives more with my general philosophy.
Personally, I take my relationships serious as a heart attack. People's feelings and hearts are at stake and I never take that lightly and I expect whoever I'm dating to be the same. If they are just doing it as a fling or just to "try it out" or on a lark, that's really hurtful to me and unfortunately, those people exist. They'll date and sleep with women but only really see themselves in serious relationships with men. It's really not that uncommon and I, for one, don't want to start dating someone that has zero potential to grow into a serious relationship. I don't want to put forth the effort of getting to know someone and, for lack of a better word, wooing them and possibly developing feelings for them, if they are already planning for it not to last because that's a recipe for heartache. Not that I expect someone to be committed to long term right off the bat, but they should at least be open to a long term relationship developing. I would never start dating someone knowing full well from the start that they aren't someone I'd be with long term because that's not fair to them and a waste of both of our time. I've had numerous people start dating me who knew, for various reasons, that there was no long term potential but they didn't really let me in on that and I ended up getting hurt. I think a lot of lesbians see a higher risk of this with girls who also date guys because so many of them plan on eventually settling down and/or marrying a man.
This is without even getting into bi girls who are in relationships with men who want to have a fling with a girl, which I've also encountered, and the girls who just want to have a threesome. Not that there's anything wrong with threesomes, or even three-way relationships, but most lesbians aren't down with FMF threesomes for, hopefully, obvious reasons.
All of this boils down to the unfortunate fact that a few have sort of spoiled things for the many. Not that all lesbians shun bisexuals, far from it. Like I said, I try to keep an open mind. But a lot of us have been hurt and that can make some people jaded, and some get tired and frustrated of running into the same thing over and over. Mostly it goes back to bad experiences that resulted in bad impressions that unfavorably color future interactions. They key, in my opinion, to bisexuals dating lesbians is to be very open about expectations and possibilities. For a bisexual who is equally open to a relationship developing regardless of gender, it's important to let prospective romantic interests know that you're serious, not just trying it out or doing it for fun. less