I'm saying this under the presumption that you have a gf... forgive me if I have the wrong impression
This is something that REALLY should be discussed with your partner, imho. It's not just a blanket rule that can be generalized to all relationships. Judging by the ever-so-wide difference in opinion on this thread alone, obviously people have some very different thoughts on the matter. So, who knows what your gf would think!
Just have a nice chat with her, basically "just in case it were to come up". She might wonder why you're asking out of the blue, yes, but would probably appreciate the open communication. It might help prevent awkward situations in the future, and who knows, maybe both of you will be cool with it!
-Verity
I personally think the blanket rule would be that kissing another girl if you are in a relationship IS cheating unless your gf has said it's okay. I've never met a girl who would be okay with this as I'm sure as hell that if I had a gf that kissed another guy I'd break up with her right away and go ape shit at the pair of them.
Surely this everyone's opinion right? I can't see any other logical way to think about this. The whole point in a bf/gf situation is that you've found someone who you'd actually give everyone else up for. Anything other than that and you just have a fuck buddy.