By that standard, @Simone_De_Beauvoir, we are all cheaters. For whom among us has not ever fantasized about someone not our spouse? That takes mental energy away from your spouse and, I guess, you would consider that cheating.
No, internet relationships are 90% fantasy. Indeed, our relationships with real people are probably 50% fantasy. Fantasies, in my book, are not cheating. It is only a credible threat of loss of companionship that is cheating. Clearly fleshly contact of an inappropriate sort that is real cheating. But it is not clear at all where fantasies of fear of loss cross the boundary into credible threats of loss.
I am aware that most women consider mental relationships more threatening than physical ones, but they are pretty much ignoring many inconvenient facts. If this were the real standard, then women would keep their men completely isolated from anyone else and any media, lest they have a thought that takes them away from the woman. This is obviously absurd, so clearly many women are not consistent in this.
You might better argue that it is cheating because it is more likely to take a partner away from you in the real world. Cheating is a word that people seem to use for fear of loss. It is an emotional term, but, I think, has little relationship to reality. Common sense, of course is quite common, and far too often, the evidence does not support it.
What are the things that take people away from their SOs? Let’s see. Work. Hobbies. Food. Hanging out with friends. This is clearly the wrong standard. Yet is sure seems to be a common standard. We do not live in an enmeshed world, I hope. Judging by the responses here, people want their SOs around them 24/7.