Many "swear words" have come to be associated with men exclusively, even though they're technically "gender-neutral". There are probably just as many female bastards, in the literal sense of the word, as there are males. But nobody ever calls a woman a "bastard" as an insult. Same with "asshole". From a biological standpoint, there are almost the same number of female assholes as male assholes, but I'd never refer to a woman as an "asshole". Especially since "bitch" usually works just fine.