W I know what's going on with you and the OM and I will not live in an open marriage. Then listen and validate where appropriate. She will most likely deny it. You will stick to your guns calmly and say that you know what is going on. You don't tell her how you know just that you know. If she doesn't admit it and want to discuss it at that point I would leave the house and not come back for awhile. Eventually she will need to discuss it and you will just stick to your boundary. The goal is for her to end the A and reconcile or give you an amicable divorce.
