"He wasn't playing a virtual game simply because there were real live other people involved."
I think that depends on who they are pretending to be when they are in the virtual environment, remembering that some people use an imagined persona, even a different sex.  John Smith-the-wimp-with-no-girlfriend becomes Rex Xavier-the-hunk-with-a-thousand-conquests-and-the-trophies-to-prove-it.  Miss Margery Swinton-the-ugly-duckling-and-virgin becomes Mrs Natasha de Veer-the-ravishing-beauty-and-sex-Goddess-who-has-seduced-a-thousand-men.  They live their dreams on line not as themselves but as their fantasy characters.  They are like actors on stage or screen, their stage being the virtual environment while their on-line "lovers" are also actors in the same play.  The difference from performing actors is only that they have no audience: They are not their real selves.  So, who is cheating?  Margery or Natasha?   

Isn't this like playing cops and robbers, where kids pretend to each other?  Robbery and murder are not the real intent, just a game about a serious subject.

(Incidentally, it is not just "cyber sex" where the fantasies arise.  I am a moderator/co-owner of a writing group on Yahoo.  We occasionally get fantasists there, too:  People who drop by with claims of being a best-selling author or - my favourite - we have a guy who turns up every so often claiming to have been busy on the script writing team of CSI or one of the TV cop series.  And then, there are the infamous trolls who turn up on places like Alternet, who posture and provoke others with inflammatory views that they do not really hold, pretending to be who they are not just for the fun of upsetting people.)

However, if John Smith posed as himself and had an on-line "adulterous" relationship with Margery, also as herself, then I suggest that scenario is very different from that between fantasy personas Natasha de Veer and Rex Xavier.  The first really is cheating, but the second is probably not.

Am I onto something?  Comments please!  :o)