No more explanations. You caught them on act. Break up with him/her. No need to say “what if ?”. May I remind you that your partner is a douche bag. So better well, break up, look for another and move on. There are billions of people in the world. That’s just one douche. It’s never your loss, he/she is the one who messed up. And always remember that karma is on it’s way to kick your ex’s butt.
Actually, I’d prefer it if you won’t. Why? So that I can spend the rest of my life proving to you how much I do. =)