What you should do is make up a hypothetical couple, and make this couple(maybe someone you work with's friend) have the same situation. Let your boyfriend know, in no uncertain terms, that YOU feel that this behavior is indeed cheating, and that YOU would not allow it. That way, you put the bug in his ear, without spilling the beans about your snooping.
I know that you don't want to leave this guy, but he is cheating on you all the same. And it's a matter of time until he cheats in the flesh. If he hasn't already.
You say the "sex is great". How do you know that he ISN'T thinking about these women while he is with YOU?
Most women would have already left him by now.
This is indeed addictive behavior.
Go find you a man that doesn't turn you into some detective, and give you a complex.
This is not how considerate people, in loving realtionships, act.
He is living a big ole cheatin' lie.
The world is full of guys that know how to treat a woman. Go find you one.
Good luck to you.