Is flirting a bad thing?

A funny thing happened to a good friend at work yesterday, and it made me think of this question; if you’re in a relationship with someone, is flirting with someone else a bad thing?
If I’m married and I flirt with someone, is that bad?
If I’m dating someone, and I flirt with someone else, is that bad?
Are certain forms of flirting more acceptable than others?
Finally, is flirting ok as long as your significant other never finds out?

I’m sure my views are different from a lot of people’s, but I think there’s nothing wrong with flirting, even if you’re in a relationship. But if my wife finds out, how can I convince her that the flirting is totally innocent?

Mr. Pu