What I would do is print out all of the correspondences and keep them to confront him when the times comes but I would ask a friend to take you to one of the meeting places with these women the next time they set up a meeting and the time and place have you friend take you there and be waiting this way the proof (the woman) will be with him. Before you do all of this you have to have a plan of where you are going to go or how you are going to get him to leave if that is what you decide this you have to think about first are you going to take him back or are you going to leave him or have him leave? If you do not have a friend then take a cab but act as the other woman make plans to meet at a hotel you go to the hotel first and set up everything like it's going to be a romantic sexual encounter and tell him the door will be unlocked and then have all the correspondences laid out on the bed and be waiting in the bathroom and when you hear him come in confront him with the proof.
One thing you have to realize is do you want to be with someone you can not trust? How are you going to be able to get that trust back? You can not let this go you have to show him that you know what he has been doing and that you even corresponded as one of the women so that you could have concrete proof that he is cheating. It really doesn't matter how you found out what matters is that you found out and have proof and he needs to be confronted with the truth and you can not do that without setting him up as if he is meeting a woman off the internet if you just confront him at home he may lie and say he never met them this way if you actually shows up then you know he has done this before.
Afterwards if you want him to leave be very clear about what you want and what the next steps in your relationship will be and make sure your friends stays there with you in case things turn ugly, you will have a support system there already. Usually I don't believe the saying "Once a cheated always a cheater" but if they do it over and over again more than likely they will continue to have this behavior. Do what is best for you now!
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