I'm not really familiar with the cyber sex thing. I guess to answer the question if it's cheating you must determine if you view this online relationship a romance? Have you avoided spending time with your bf to chat with the online guy? Do you feel the need to hide the nature of your relationship with the online guy from your boyfriend? Would you think your bf was cheating if he had a similiar relationship online or in person with another woman? If you answered yes to most or all of those questions then it would be a form of cheating. The same as going out and having sex with someone, probably not, but I'm sure that your boyfriend finding out about this online thing would be very hurtful to him.

The fact that the online guy makes you uncomfortable with his flirting after you've already made it clear that you're not open to that type of relationship speaks to the man's character, and not in a good way.

Like I said, find ways of spicing up your real life so that you're not lost in the fantasy of online. Cheating is very rarely just about sex, there is something lacking in your real relationship, try to figure out what it is and fix it. You'll be much happier in the end.