I agree with Nick8's assessment that your boyfriend's opinion is the one that really matters.
OP, you could have a hundred of us here line up behind you and agree that it's not cheating, but when your boyfriend finds out that you're lying to him about what he considers cheating, it won't make a lick of difference.
Worse, regardless of whether what you're doing constitutes cheating (which, by the way, it is, as it violates the parameters of your relationship), you are lying to him. You are lying to your boyfriend. You have been lying to your boyfriend for years. If he were asking me for advice, I would tell him to dump you and move on to someone who can give him what he wants.
And, frankly, unless you feel you can stop violating the agreements upon which your relationship is based, I'd advise you to do the same. Move on to a guy who doesn't consider cybersex with strangers cheating, and let your boyfriend move on to someone who does. Or at least come clean and give him the choice he deserves.
I don't say it to be mean, but because I think it's kinder to end the lies and the grief that will inevitably follow.