To me, cybersex isn't really cheating in the common sense of the word. It can be, but it doesn't have to be. It really is masturbation, with the exception that it takes another person. It can all be a lie, a fantasy, no different than a picture online.
I can be cheating, however, if your relationship with your online lover becomes more impotant or more fufilling than real life.
I guess what I want to say that cybering can easily be an innocent activity--but it can also easily be infidelity.