My thoughts on the topic - cheating is doing something with somebody you wouldn't do in front of your spouse, and/or you know they wouldn't be okay with, or otherwise impacts their ability to give/receive something that is supposed to be shared only with one's partner.
Just like the virginity debate, it's all in how one views things, and it's not black and white.
And whether we admit it or not, there are certainly varying degrees, or levels, of cheating, and we all have our different boundaries of what is forgiveable and what isn't.
For me, personally, if it's something I wouldn't say or do with my wife right next to me, then it's not okay. It's not necessarily "cheating", per se, but it's in the same conversation for sure.
As some of you know, my wife used to be rather flirty with others, early into our relationship. She never crossed lines, from what I saw, however she admittedly liked the attention (don't we all?) and it unfortunately gave some people the wrong message. I wouldn't say she invited it, however she didn't shut it down, either.
She, of course, didn't see things the same way as I did, as is normal. I told her to look at it from my POV, that my (then) girlfriend is acting as though she is available when she's not. How does this make ME look? How does this make HER look? "But I'm not available, don't you trust me? They know I have a boyfriend." "I do trust you, and I know you're not available, but I don't think those guys do." "But it's only flirting." "To you, yes, to them, I'm not so sure. Are YOU sure?"
But more importantly (to this topic here) is that I asked her to think about whether she would be accepting of this type of behaviour if I were standing right next to her. If some dude were to make a comment about her, say, boobs, and I was right there, how do you think I'd react? Would you respond any differently in this case?
So basically, if it's not something you'd say or do with your partner right next to you - don't effin' say or do it. Is it *cheating*? Probably not. Is it okay? Absolutely not.
But then again, the word "cheating" denotes one gets something from somebody that is not their partner, that is otherwise supposed to be something one only gives or receives from said person. If my wife (then girlfriend) was receiving comments that were boosting her ego, making her feel good, or desirable, from people other than myself, is that not cheating me out of the ability to do so?
We all draw our own lines, though, and that's okay. For some people, porn is not okay. Your partner is getting turned on sexually, and getting off to somebody else who isn't you. Strip joints, even if there's no touching or lap dances. That sort of thing. For others, one or both of those is perfectly acceptable. My wife doesn't watch porn, but it wouldn't bother me if she did. She also doesn't go to strip clubs, but that WOULD bother me.
I think the only universal agreement on what IS cheating, is intercourse. Oral or manual sex comes in a close second, but clearly some people don't view that as cheating (sigh). Making out with somebody, slightly below that. And so on, and so forth.