I get not wanting guys playing a big part in your yuri - I've complained about that myself, even - but bisexuals? Really? I think I'm starting to understand why so much yuri takes place in girls' schools where nobody has a history and men are a hypothetical concept at most. Really though, is it that hard to just pass over a story if it's about something you're not satisfied by? The tag is right there, it's not like you can miss it.
It's funny though, you don't get this demand for gold star purity with yaoi. There, most stories involve a "straight" guy who discovers his love for dick, and I never see comments complaining about how it's spoiling the genre. If anything, the conflict between gay characters who can only be attracted to the same sex and can't legally have a family with someone they love versus the "straight" characters (actually bisexuals) who have the option of moving on and happily marrying is probably more prevalent in yaoi than it is in yuri, and yet I still don't see this kind of rejection of characters who aren't sufficiently pure. My guess would be that the difference has a lot to do with how women are seen as "tainted" by their associations with men in a way that men aren't with women, and that the distrust of bisexual women is partially an expression of this broader hangup, but that's only a guess.