You shouldn't cheat on your partner with their parent.
Gary is dating Monica, and her mother Tina is flirting with Gary while Monica is away.
Gary wants to respond to the flirting.
Gary tells Tina that the flirting is inappropriate and that he'll tell Monica if it continues.
Gary flirts back at Tina, and the two end up having sex.
Tina backs off, and Gary and Monica continue dating for a while.
Monica finds out and breaks up with Gary and cuts her mother out of her life.