I wouldn't make a big deal over it personally (I would try to use the proper pronoun), but if you're a trans girl and someone call EVERY girl "bro" is it really that bad?
Yeah, it is. Trans, genderfluid, and agendered people gotta deal with erasure and being misgendered all the time. By checking what people's pronouns are, you're helping them become comfortable AND you people one small part of the road to trans/genderfluid/agender acceptance.
