Most Helpful Guy
Tell him you got HIV or some other STD and you are not sure if it was from him. Ask him to get tested... Its important that he does not spread it to the other women in his life... (I'm kidding.) Never lie. Just tell him you don't want to sleep with a guy who sleeps around.
Most Helpful Girl
He does it because you keep responding. Tell him your limit and what you will not tolerate. Stop giving him chances. Stop talking to him