I've had a few bisexual people tell me they don't understand how people could be homosexual/heterosexual or that they don't believe that true homosexuality or heterosexuality exists. (They've been largely into the trans stuff as well tbf so i can see "but wahh you can't tell gender from looking at someone")

What you said about choosing an identity and a "group" to run with also rings very true. I think that's especially a problem with younger people, and especially on the internet where there's a pretty insular and echo-chambery view of most of those "communities"