To me, this FML translates to, "Today, I found out I've been having sex with my mom's doctor." The things a gynecologists do with patients are strictly professional. He and your mother are not having sexual relations, OP. You are an adult, (I hope, since you're having sex with a man old enough to carry a medical licence), and I really don't see why this is bothering you. But if the thought does make you uncomfortable, by all means, break off your relationship.