>I think the contempt is there, because we're talking about someone's parent

But I feel that this is more because the mother is the more 'revered' parent? People often hate their father, but it's far rarer for people to hate their mother. Cursing someone's mother is, to me, just a way to hurt someone by insulting the person they love the most.

This of course would imply the exact opposite of what you're suggesting.