You're pretty much right, Bee-Dub, but I have heard a woman refer to another woman as an asshole on occasion. It's not common, but it can happen.

Perhaps one reason that many common swear words are conventionally directed only at men is simply that for a lengthy period of Western social history women were not expected to use those words at all...nor were those words to be used on them...it was considered improper.

Those days are long gone, but a subtle effect may linger on in people's instinctive use of language.

What about "bastard"? It's hardly ever used with the literal meaning (born out of wedlock) intended any more. It's normally used nowadays simply to indicate that one finds a person unlikeable, undesirable, unfair, dishonest, etc...in other words, as a measure of his character, not his birth status.