Being Upset Is Different from Controlling Someone
Being upset and unsettled by someone's behavior and letting them know is very different than trying to control someone.
For example, if you were to say to your boyfriend: "The nudie pictures you get in email make me really uncomfortable and make me unsure of how faithful you can be to me" is different from saying "I insist that you no longer receive any emails".
Likewise, saying, "I do not want to be in a relationship where there is a high risk of infidelity" is different from saying, "You better not cheat on me" - the first statement in each pair is about how you feel and your reactions, the second are telling him what to do.
You Can't Control Him
Which leads to my whole philosophy on relationships: you can't really control what someone else does, you can only control your reaction to it. And in this case, your fear that this guy may not be faithful to you and even trying to figure out what fidelity means (e.g., does it include cyber-sex?) are VERY rational responses to his behavior.
Consider Being Single
I suggest you talk to him immediately, and weigh the pros and cons of putting up with behavior that makes you feel rightfully nervous rather than being single and not having to put up with anyone else's behavior at all. Also, try some steps to improve the self-esteem.
Saying No Is Hard
Just because we can't control someone else's behavior does not mean we should put up with it in our lives - but saying "Not for me, thanks" is going to take some work on your part.