"Multiple places" is not a source. Where have you heard it? What kinds of places? From what kinds of people? Some examples would help.
Well, I've never heard it as a direct insult, I just see it on stuff like people talking about shit people say to bisexuals.
It could be they've had bad experienced with men. There are straight women who hate men, most straight men fantasize about lesbians. Upon finding out their partner is bi, men probably try to talk them into threesomes and the idea to the bi women is vile? A lot of lesbians who are figuring things out identify as bi, and maybe some of them have created that perception? Obviously not all lesbians hate men, but you know some do. Idk, sometimes people just hate who they date. I've known some lesbians who causally say they hate women.
I don't think that there's really any logic behind this, it seems more or less exactly the same as most stereotypes.