What Exactly Constitutes Cheating?

This is an interesting ethical question. When is something cheating and when isn't it? Let's take a look at Brad Pitt. King Knee Pads himself actually asked Brad a tough question. The name will still stick though. Larry King asked Brad about when his relationship with Angelina Jolie began. Brad responded by saying, "Well, that [romance] came after Larry. That came after."

For the sake of argument, let's just choose to believe Brad. Brad left Jennifer Aniston shortly after the film was completed. If you are flirting with someone on a film set or at work or at the grocery store for several weeks, and as a result of that flirting and getting to know that person, you leave your wife, and then begin a relationship with the subject of your flirtations is that cheating?

Does there need to be intent? The American Heritage Dictionary defines flirting as:
1.To make playfully romantic or sexual overtures. 2. To deal playfully, triflingly, or superficially with.

So to me it seems that you can flirt without meaning anything by it. Everyone does it whether they are in a relationship or not. Did the flirting with Angelina make Brad realize that he wanted something more and he had no expectations of having a relationship with Angelina? If so, then maybe he is in the clear. But, if Angelina made it clear to him that she would be open to a relationship with him, and he continued flirting with her, then at some point that relationship has moved to a new level.

I think we all agree that Brad at some point was cheating, even if it was just mentally. But, what if his flirting with Angelina just made him realize he needed something more than Jennifer, and he left her, but for someone other than Angelina. We wouldn't consider that flirting he did with Angelina as cheating, so why do we consider it cheating because he did end up with her?

I think he cheated. I'm just trying to see what everyone else thinks. It doesn't have to be a movie set. It could be someone at work. It could be anything. Where is the line drawn? I think the line for me would have to be this. If your significant other saw what you were doing, would they be okay with your actions or words? If the answer is no, then to me it is cheating or at least being dishonest.