The whatever reason bring that they’d have played themselves because homophobia.

I imagine the potency of yo mama jokes that focus on banging her, as opposed to accentuating how ugly she is, comes from the discomfort of sexualizing someone that you want to not have sexualized given your intimacy with her. Reframing your mom as a sexual being rewrites the experience of affection with her, for example.

The ugliness piece devalues someone precious to you.

I also thinks it’s just a rules of the game that people decide on and then forget. Participating in a game about who will gets upset, everyone implicitly agrees that this is something they can fall prey to having a reaction about. Sort of social game theory