That’s not really how yo mama jokes work, in an insult sense. They work by implying that suddenly you’re, socially speaking, their father, the rest is flourish. Like the maid uniform your mom wore last night.
These types of jokes “work” because people consider it an insult when you somehow suggest their mother is promiscuous, or fat, or whatever. When you wonder why that’s sexist ask yourself why these kind of jokes aren’t told as: “Me and my friends gangbanged your father.”
They do happen, but you’re right they’re usually centered in bigotry. I think “banged your mom” is more of a dominance play, as iirc there have been times where women have used “I fucked your dad last night” in a similar way, but it strongly skews the other way due to toxic masculinity and homophobia. Saying I fucked your dad last night as a man talking to another man would be suggesting the father is gay or bi, and that your yourself are as well.
Why would anyone care about that? I already explained how they work: A new man moving into a mother-son relationship is not exactly a trivial thing, you’re completely ignoring even the concept to then assert, without argument, that it must all be due to sexism.
