I've found a lot of women don't believe men can be "as" bi as women can be, and there's this suspicion that if a guy has ever been (or just wanted to be) with another guy, they must be more into guys than women, and wen don't want to have to wonder/worry about that. I think this is just a reflection of societal implications around male and female gayness being different though, that women are accepted to exist on more of a spectrum (perhaps because attraction to women feels natural to straight men, so it is less "surprising" or foreign to see even other women feel it), while men are considered more binary.