There is NO REASON to confront him. You are not in an R with him in which you need to work through the infidelity and trust him not to cheat again. THERE IS NOTHING TO WORK OUT WITH HIM ON THIS ISSUE.
The start date of his A is irrelevant to your current sitch. You are parallel parents. Period. There are legal matters to address. Period.
It means nothing about the kids. The kids had a father yesterday who was having a PA with the neighbor. The kids have a father today who was having a PA with the neighbor.
There is also the question of STDs which I think you addressed, but I'm not sure. But, that is between you and your doctor. There is NO REASON to discuss the risk of STDs with H. You will NOT get trustworthy info from him.
Keep the fact that you have this information private. Do not tell BFF. Do not tell anyone right now. It may prove to be a useful trump card at settlement time. But, that is its ONLY use to you right now. Don't show your hand.
