> I think the contempt is there, because we're talking about someone's parent, and for some reason, all cultures (that I know of) picked the mother as the one being out of line. As in, when there's someone that's out of line, that has to be the woman.

There's definitely contempt there, but just for the person being insulted.

I think you're analyzing those kinds of insults using the wrong framework. It doesn't have much to do with women as a class, but rather with the relationship with the insulted person to his mother. Insulting someone's mother will probably get under someone's skin more than insulting their father, because (especially historically) they probably have a warmer/closer relationship with their mother due to gender roles around child-rearing.