This argument is entirely subjective and depends upon the boundaries (or lack of) that each couple (or trio?) defines and delineates in their own way.
In other words, there is no "right" answer...only the best answer for your own situation.
In my world, I define cheating as any sexual gratification that I am not aware of. In other words, do I know what's getting my boy off? And assuming my knowledge is omniscient, am I ok with what I know? Those are the questions I ask myself when forming boundaries for my relationship, and those are the questions I communicate to my BF. So far, it is healthy and works well.
I won't get into what is and is not "ok" with me (ie porn, webcamming, etc.) as that is where subjectivity most definitely dictates the agenda. I might as well talk about politics or something...