Dear LostTrust:
It sounds like you and he have a lot to talk about. I think everyone fantasizes about seeing someone else. That's why being faithful to one person is an act of the will, a choice. I'm not too concerned if a guy just looks, but having a separate email account to receive matches indicates that this has gone further.
I suspect he is having some second thoughts about your relationship. Sometimes, even when we have a good thing, we begin to take it for granted and wonder "What if?" The only way to address this is to sit down with him and lay the cards on the table.
The issue here isn't that he was looking; it's that he was being deceptive. I think you need to share with him how this made you feel and how it is very important for you to be able to trust him. Talk about your feelings, not what you think he's feeling.
We all make mistakes. He needs to know that you're concerned about the deception, not the fact that he was curious. He will either learn from this and correct his mistake, in which case the problem is solved; or he will just try to be sneakier, in which case he will get caught and it's time to tell him to have a nice life.