It might simply be what part of the country I grew up in (southern CA), but I never "bitch" used in that way about men. It had an entirely different meaning, similar to serf or 'gofer' -- whereas when people used it about a woman, it basically meant the same as "asshole".

I hardly believe I've typed these words out. Blech. In any case, I think that for a large part of your audience, and perhaps US society in general, "bitch" definitely has a gendered connotation -- even if you did not intend one.