The only person who is talking sense in this thread is Chizip.
If you set up a fake email, there are two possible outcomes:
1. He answers it with "gee thanks for responding, but I'm sortof involved right now, in fact I was just about to delete this ad". In this case you now know the truth, he is the man of your dreams and you can feel alot better about it and never mention it again. When you see him again, give him the biggest hug and kiss and if he appears startled, just say something about how you feel so glad to be with a genuine person such as him. He needs to know nothing, and probably wont care as you are hugging him so tightly.
2. He answers with anything else BUT 'gee thanks for responding but I'm sortof involved right now', and even suggests a coffee meeting, he is a player and you can then decide if you want to stay with a player OR you should break up with him immediately with absolutely no explanation.
In case #2 he deserves nothing from you. He doesnt deserve honesty, trust, maturity or any of the other lame assed reasons I've seen so far why you shouldnt exercise your right to investigate his behaviour. It also does NOT matter if he even finds out it was you, because his opinion of you IS COMPLETLELY WORTHLESS in the case that he answers the email.
I'm not saying arrange a meeting where you get to shock the hell out of him and confront him. Im saying you do right by yourself and find out what he's all about without him knowing that you are investigating. It is fully within your right. Then you make the decision if you want to spend time with a player.
And if you do cut him off, why should you give him the benefit of a lesson learned that he just lost out on a great relationship because of his online cheating? Cut him off with no other reason than you just want to remain friends. Let him wonder, let him sweat. Let him go through life making the same dumbassed mistakes, if he wants to ruin himself its not your responsibility to teach him how to behave.
Then again, you could just confront him about it. Sure he may give you all the reassurance and words you want to hear. In that case you'll never REALLY know if his answer is truthful and sincere, or if he cooled off a bit and then set up his OWN fake email and profile in a week or so so he could continue playing. You will go through life never really knowing. NEVER.
Think about it.
All's fair in love and war, they say.