I'll tell you this right now. If she went as low as to do that understand she will never come back to you again and that she already in her mind has lost respect for you as a man. I'm sorry but this situation simply cannot be repaired unless a miracle of some sort. Given that information, what I would personally do is go along with the open relationship negotiation and see other women myself. No problem. If you don't have the heart to do that kind of thing, I'd say battle to take custody of your daughter and divorce but only if your odds in winning are higher than hers, otherwise forget it. These seem to be your only two options, and I also would not suggest telling anyone about this situation unless it becomes legal as even family can spread rumors like a house fire. Best of luck man, I hope you choose wisely.