In my opinion kissing and beyond is cheating.
It's not particularly nice if your other half is flirting with other people, but it is harmless. It's only when it goes beyond flirting that bad things happen.
My ex-g/f is really bubbley, bouncy etc, with a lot of male friends, most of whom would shag her silly given half a chance. What she sees as just messing around/having fun, being mates, is seen as flirting by other people. They were flirting with her, but she wasn't intentionally flirting back. It would be a bit harsh to say that constitutes cheating.
Looking is natural. Wanting to do something is bad news. Doing something is cheating.