I once had someone ask me why "dickhead" was not a sexist insult if "cunt" was.
I think I have figured it out.
Almost every other insult that can be directed at a man doesn't actually insult him. It insults the women that are "his" in order to insult him and it does so by applying some of the same reasoning that makes calling someone feminine or a female body part insulting.
Case in point, the insults "son of a bitch" and "mother fucker."
The first is pretty obvious. You aren't insult the man on the basis of his gender, or using his body as a negative, insulting thing, you are insulting his mother.
The second is a bit more complex, but it implies that a mother is not desirable, or sexual and therefore there is something bad about having sex with them. Maybe it is intended to imply that the man is less than because he can't get anyone "better" and is a reflection of our culture's obsession with virginity and how that is a measure of a woman's worth. It could also be calling up echoes of the Oedipus mythos, implying that the man in question fucked his own mother, who cannot possibly be desirable because she is "old" and a mother and was supposed to have turned in her libido when his birth certificate was signed.
This is why I stick with "asshole" as my catch all insult.