The language in this post might put some people off. Consider yourself warned.
There are some terms that are used as insults that need to be either defined, or perhaps redefined. There is a line between terms used to show disapproval, versus terms that show a lack of respect. Not all insults are equal, there exists levels. Some are situational, some are universal. But lets be honest, all are great fun when used right.
"Asshole" is one we hear a lot. This one is used so much, and is so descriptive of so many, that it has lost all impact. In fact "asshole' is practically interchangeable with "person". While true that not all people are deserving of the title of "asshole", at the bare minimum 997 out of 1000 are. Man, woman, or other, the term "asshole" has lost all bite.
Now to talk about terms for women. "Bitch" is one that comes out when someone is mad at a women, you might disapprove, but generally there is still some measure of respect. "Cunt" on the other hand shows that you have lost all respect for the woman in question. Your boss might be a "bitch", but the girlfriend that cheated on you is a "cunt". No offense intended to any lady reader I might have.
"Bastard" and "douche bag" work for men about like "bitch" and "cunt" do for women. While "bastard" did once held meaning referring to children of unmarried couples, that use has fallen almost completely into disuse. "Douche bag" (often shortened to "douche") is an ironic insult, as it is an item that literally cleans vagina. However, when a man is a "douche" he is getting no where near any vagina.