LITB - I am going through the same thing right now. The last week of my sitch involves child custody. I would be very interested in the responses here.
After discovering my W's affair, she pleaded that I not take our S away from her. She feared me taking full custody (which I discovered to be a very real option when combining my conservative county and my W's same sex affair). I told her that I would never do that and agreed to 50/50 custody for the present and agreed that if we were to ever divorce, 50/50 would be agreed upon in mediation.
Fast forward two months, W is not liking the 50/50 arrangements and wants to meet a "parenting coordinator" to see what a professional might say is the best arrangement for an infant. I was not born on the moon. The pro is going to go with single physical custody with lots of visitation for an infant. This is something I am not okay with nor did we agree on.
Our last couples therapy session got pretty ugly when this was discussed. The conversation was making it very difficult to DB and validate her feelings.
I received a few responses to my dilemma. One was to protect my custody of my S, since the current situation sets a precedent if D would be in our future.
Looking forward to others chiming in. This is a pretty big boundary to set and still continue with the DB.
Best of luck to you.
Me - 33 W - 33 S - 9 months M - 3 years T - 5.5 years Bomb - 12/14/10 ILYBNILWY PA discovered - 1/18/11 PA began - 3/22/10 Separated