People are willing to blame anything but themselves.

"Oh it's because my ex was bisexual and they couldn't be happy with me because they're bisexual it's not because I constantly ignored the stuff they said was a problem for them"

Bisexuality I think is a big red target when it comes to this. If a bi man dating a woman leaves her, her and all her friends can be like whisper whisper "he's gay we told you he's gay" and it's like "confirmed" if the next partner or sexual action is a man. And also I think this happens because it's way easier to hop on Grindr and hook up with another dude when you're single than find a casual hookup with a woman. Gay casual sex is honestly just super quick and simple, and if you've been with a woman it's actually kinda fun to be like oh haven't had this in a while. But of course it never means you wouldn't stay in a monogamous relationship with a woman, just if you want something casual and quick... Grindr.

For a bi woman, it's probably similar though I can't say personally. People want to think that it's something out of their control, something they couldn't influence, some supernatural power that prevented them from being a better person and making it work. Bisexuality is that supernatural power, some affinity towards penis/vagina, some body part they don't have the power to wield.

It's just easier to say that a partner doesn't like men/women than to say they didn't like you. Bottom line. It's not my fault, it's them.

Unfortunately this leads to a lot of shitty stereotypes for us, but you know what, shitty relationships are going to be around forever and it's better to be out of one where your partner thinks like this than stay with them for life and have them never want to work on problems and blame other things outside their control.