I'm going to throw a proverbial monkey wrench into the works here:
What if a person is engaging in cyber sex posing as someone else? Is it still cheating? Here are a couple of examples:
I have a dear friend out in California who earns her income working for one of those phone-sex lines. She is a sweet girl with a delightful personality, and a soft, voluptuous voice. However, she is handicapped, quite obese, and (I hope she forgives me for saying this) homely. She has no intention of ever meeting any of the men who call her, even though she has several "regulars" who have been calling for years. It is her only means of income, and she makes a fortune. She bought a house last year -- something she could have never done on public assistance, since she cannot otherwise work. She is in a long-term relationship. She considers herself a performer, an actress. She and her boyfriend are in agreement that she is not cheating.
Another example is a man in his mid-fifties I know in Canada, who poses as a 23 year old bodybuilder. He believes that most of the people he has cyber sex are not representing themselves truthfully either, but rather are play-acting fantasies. If any of his cyber-partners ask for his photo, he provides pictures of himself from thirty years ago! Again, he has no intention of meeting any of these people face-to-face. He believes that most of his cyber partners would not ask for a face-to-face meeting, either, simply because they are not in reality who they say they are. Is this cheating? I don't know.
The only people I would pass judgement on would be those who deliberately misrepresent themselves with intentions of having a face-to-face meeting with their cyber partners in an effort to tantalize them into a sexual encounter. I know several people this has happened to, including a couple who travelled very long distances to meet someone who turned out to be an utter fraud.