"Have some Oyakodon" one cracked me up because her mom is still pretty good looking in basically the same ways as Megumi, and that can be slang for having a threesome with the "parent and child."
@AsuraRaza It's more or less a double entendre rather than an expression specifically for it. The name "oyakodon" basically means "parent and child bowl" because it's chicken and egg on rice. So people adopted it as a term for nailing a parent and child at the same time, probably because it's a fairly common trope in porn.