I've mentioned before the time I was outright shunned on a game because I wouldn't 'make' my straight male character gay. That since I was a female player, it was expected of me and since I wouldn't do so for the people asking, I was labeled homophobic.
He was straight. 'Making' him gay felt even more insulting, IMO. But I got outright shunned out of that area (a Weyr on a Pern game) for not doing it for them (they thought he was hot, so...).
I've had lesbian chars and had other women approach me asking me to 'teach' them how lesbian sex works (do u not google?!). This is what led me to keeping a lot of my chars' sexuality under my hat as it were.
The most recent case that I would def. label 'bi-erasure' was a character who was very... protective of other women, but interested in both men and women. When she flirted with a guy, he had this whole thing of 'but isn't she a lesbian? She's only into chicks, right?' and seemed utterly and completely baffled that she could be interested in men, too. He largely acted like I was making it an OOC thing (as if I was interested in him, the player, vs the characters).
I find that's how it 'works' most of the time on the bi-erasure front. Whoever people see your character flirt with (or even BE flirted with) first is what they assume of their sexuality. Not, I suppose, unlike the real world.