How bad is this, in general? Is it something you just overlook or the kind of thing that you have to stop and make a big decision?

I know you can look at this in terms of what the person's intentions, whether they wanted it to go anywhere or it was just harmless flirting to them, but for the sake of this question, how bad is the acting of flirting in and of itself, with other people while you're in a relationship? Does it warrant a breakup? Or at least a fight?

Thoughts?