I think cheating is whatever your
so thinks it is.
If it is ok with your significant other that you flirt at work and go to lunch with people, then it should be ok.
On the other hand, if your so doesn't even want you looking at other people (drop them), it isn't ok.
My wife doesn't mind me going to strip clubs if I want (i don't). I actually went to a "lingerie show" last weekend that had several models wearing skimpy clothes. There was a playboy model, Laurie Wallace, signing her issues... My wife said "That sounds like you will have a good time! Have fun, sweetie!"
Cheating could also be defined as what hurts your conscience. If you feel guilty about something, don't do it (again).