Guys never call a girl a bitch without either sounding extremely camp (something most guys don't want to do) or sounding offensive. So there you go. That's why girls always take offence to it.

Of course, if you're good friends with someone, you can generally get away with calling them all sorts of things regardless of their gender. Likewise, if you're not good friends with them, you should expect a slappin' for your offensive yappin'.