That's a really subjective question. It depends entirely on your own personal morality and his. If you're religious, you're pretty much fucked, cuz the Bible says that as soon as you've been unfaithful in your heart, you've cheated. In other words, all it takes is going "damn... that's a hot ass" to be a fucking adulterer. No wonder milllions of people stop listening to religion every damn day.
Assuming that your morality is not religion-based (ie, it's not retarded), then whether or not online liasons constitute cheating is a personal decision. But here's the catch, it's a personal decision that you and your partner have to AGREE on!
Being in a relationship means that what YOU think isn't all that matters. Now, obviously your boyfriend considers this cheating. That's something you maybe should've known and perhaps you should've respected it.
Ah, but you didn't write to me just for me to tell you to make up your own damn mind! You wrote in to have Lord Galen shove his own morality down your throat, because you assume that I'll agree with you and call your boyfriend a stupid cocksucker. Then you can show this to him and feel vindicated and he'll feel pwned.
Well, that's exactly what I'm gonna do, because you're right. All the shit I said about a personal morality and making the choice together is true, but the trouble is that sometimes your other half has a "personal moral code" that snuck aboard Marty McFly's DeLorean in 1955 and hitched a ride to the present.
Listen, the Internet isn't "real" and as long as you didn't try to make it real, then you didn't cheat and your boyfriend is a fucking pussy. Tell him to man up and stop whining about you having a fucking FANTASY that didn't involve him. Jesus fucking hell, how much more insecure can you GET?! At the end of the day, that's all the fuck an online encounter is; a fantasy. If you tried to meet this dude in real life, that'd be different, but from what you've told me it was nothing but an innocent cybersex session. There's nothing real about it. You didn't really fuck another guy, you just fantasized about fucking another guy and if your boyfriend is too much of a goddamn insecure fucking baby to handle that, then you need to stuff a pacifier in his mouth and tell him to grow the fuck up!
Hell, I'm married and that doesn't stop me from fucking around with girls on the Internet. My wife knows all about it and doesn't give a shit. Why not? Because she knows I'm not going anywhere and these people don't mean anything to me in a real romantic sense. It may be a friend or whatever that I play around with online, but I'm not like "in love" or some stupid bullshit like that. My wife knows that and is secure in the knowledge that I'm not shopping for anybody else.
I really don't know if my wife does any fooling around online and I really don't fucking care. That's another issue right there. See, I don't know what my wife does online because I have some fucking RESPECT for her PRIVACY and I don't go reading her chat logs and e-mails and shit. I could. I know all her passwords and she knows all mine. The only thing stopping either of us is TRUST. That's something you gotta have for a relationship to work and it's obvious that your boyfriend does NOT have it. He read your chat logs and that alone is grounds to fucking dump his loser ass!
So, what's my final advice to you? Toss that dickhead away like last week's abortions. It doesn't matter what he thinks of your online activities since he's hardly a pillar of fucking virtue himself. But if you should decide to keep him around anyway, let him know that cybering is something you enjoy doing, it doesn't mean you're cheating on him and it really isn't any different than when he jacks off thinking about Jessica Simpson (or whatever other dumb whore he thinks about). If he can't handle that, he can fuck off.