FWIW, both I and (even more so) my partner and beta, have DEFINITELY encountered this kind of biphobia in reader reactions to stories of ours that were primarily m/m but had scenes involving one of the male leads in a sexual situation with a woman, even when the dude in question is canonically into ladies. People have this kneejerk "ewwwwww, girls!" reaction, equate any position other than a Kinsey 6 to homophobia on the writer's part, and then vomit these views all over our comments. It's not a good look.

Which is not to detract from the fact that it happens in the policing of f/f ships as you cite, but it's CERTAINLY present in m/m ships as well.

(FWIW I say this as a Kinsey 5.5 more-or-less-lesbian who writes a ton of f/f as well as m/m and m/f.)