Thoughts are private. Communication is the sharing of thoughts, which can be done over the internet. Sharing lustful and intimate thoughts with another person may violate the agreed-upon boundaries of a relationship, at which point it becomes "cheating."
There's also cyber sex, which is very similar to phone sex (it's mutual masturbation while "chatting" with one another). That's more than just having "lustful thoughts," that's acting on them, in a way that's destructive to relationships. And it's cheating.
In an arbitrary sense of the word.