But I take your point that online interaction is just as "real" as one makes it. So I would amend my earier response to say this: having webcam sex is no more cheating than masturbating is. But developing feelings for your webcam partner -- the kind of feelings that could undermine your relationship with your girlfriend -- that's cheating.
I understand completely and for a man I think I'd agree. I can't speak for all other women, but I'm much more cerebral and don't think with my vagina (no offence to men, I love men) and I would develop feelings. Not necessarily intimate, but I'd feel a connection of some sort with the person I was camming with.
I don't consider masturbation cheating, that's absurd.
Mutual masturbation or my significant other being watched by someone in the same room - I'm not sure how I'd categorize this. But I think it's a step towards cheating. Same with webcam fun.
Full sexual contact - cheating
I suppose there is a spectrum. It's likely that both members of the relationship disagree on what that spectrum is. Therefore I'd err on the side of caution.
I think of how I'd like to be treated and how I'd treat my partner. I'd feel guilt if I was having online fun with others; I'd feel like I was cheating. My partner might think otherwise if they were doing the same.