I wonder how old the two that would set up fake accounts are? Is it an age thing to do shit like that? *shrugs*
Op: First off, shame on you for snooping. What you did is akin to opening up his snail mail and reading it. Would you open up his post office delivered mail? Second. Tell him that he left his mail open and you know that he is opening up his emails from the Adult HookUp site and you'd like him to delete his account. See what he says about that. If he hesitates or refuses to then you have your answer. If he has no qualms about deleting his account then it's quite likely that you can trust that he wants to keep you as his only sex partner.
If you can't trust him then you need to break up now because if you have no trust then you're just going through the motions and being angst ridden and insecure. What fun is that?
Don't play any games. Two wrongs don't make a right and setting a guy up for failure is a self-defeating shit test. Communication and trust two corner stones to a happy, healthy long term committment work your communication skills on this.
Good luck.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion