It's a lot more common than most people realize. The best thing to do is to tell him you know and work out a way to have a socially monogamous but privately polyamorous setup. Be a collaborator with the two of them to make sure they can stay under the radar of friends, family, and coworkers. Be loving and intimate in nonsexual ways with your husband. Be friendly with his girlfriend -- your metamor -- to reduce your jealousy. There are lots of good books 9n polyamory out there, some of which probably include advice for this exact situation.