A B-girl is another word for calling a person a bitch. Although, that person could not always be a bitch, so they aren't actually a bitch. The person does, however, act bitchy on the occasion so they are a B-girl. The word is meant to be less harsh. There is no change in the word B-girl for males. If a male is being a bitch, then he too is a b-girl.
"OMG, Lilo was being such a b-girl today. Ew."
"OMG, Max was being such a b-girl today. Ew."