Bisexual women are a very mixed group. Lesbians normally want a serious relationship with women, heterosexual women with men, but with bisexual women it can vary a lot and this is where the problems come from. Some bisexual women are essentially heterosexual but interested in getting some girl-on-girl experience just because it's something new and exciting for them. On lesbian dating apps there are many male-female couples with a bisexual woman who tries to catfish lesbians and tries to get them to sleep with their boyfriends in a threesome. Then there are bi women who want some short lived relationships with women, but they know that ultimately they will break up to marry a man. And then there are of course regular bisexual women who are looking for serious relationships with either sex. Some bisexual women are even interested in serious relationships with women only. The problem is that it can be difficult to tell which one of these is the woman you are speaking with. I don't think that any lesbian assumes that no bi woman is serious about dating women, it's just that some find it easier to simply not date bi women altogether than guess whether or not it's an experiment, a ''phase'' before marrying a man, or if she's really serious.