Steve Martin in All of Me where he shaves, while Lily Tomlin has control of half his body. Perhaps the same scene, or another bathroom scene, same movie, where she asks "shall I tap?"
Ellen Barkin in Switch. (Some dude) is murdered but goes neither to heaven nor hell and is returned to life by God and the devil but as a woman as a test. He wakes up as Ellen Barkin. Goes to bathroom and sees self in mirror. Freaks out for a while. As he is coming to grips with his new situation, he/she can't help but feel up his/her new tits. Hilarious scene.