My Girlfriend has sex with his old's - How do I get him to stop? If you have noticed that your girl has sexual relationships with her old man, it is time to talk to him. This is the best way to learn how to end a threesome because if he is having an affair, it means that he has shifted his priorities in his relationship with you. If he is still committed to you, he will respect your desires more and think more deeply about what you want him to do for you..