Everyone's different. One person's relationship dynamic will not be a perfect fit for someone else.
I know couples who have open relationships and what they do would be 'cheating' for another relationship. This said, there are still scenarios for them as an 'open' couple, that would class as cheating. I think the idea, for anyone, is that it's unacceptable when deception is afoot and you do something that puts your partner's emotions, health or stability at risk.
In other words, it is best to adopt the Google mantra: Don't be evil.
My take on it? Cheaters always, but always, get caught. So I don't worry about it. Flirting is different. Again, you have to feel it out. There's a difference between someone using their sexuality when they communicate and using that communication to serve their sexuality.
I wouldn't be over the moon if I found out my OH was sending sexy messages to another person. It's either an indication that they're cheating, or doing something that they know would upset me. Either way, they're not someone I want to be with. For another person, this may be ok.
I know couples for whom talking to the opposite gender at a party results in punishment. I couldn't be like this. Sure, I don't want a lover to be all over another, but I do like the idea of going somwhere, being free to charm the pants off other attractive people and then come together at the end of the night. In fact, I find it kinda hot. But then it's almost physically impossible for me to be jealous. I always figure that someone has chosen to be with me and if they don't want to be with me, they'll leave. Naturally, there's the cheat factor but this comes back to cheaters-are-always-caught and therefore, it's better to not worry about it and congratulate yourself for having a good relationship.
But then, perhaps I have little authority on the subject because I've had little experience of sketchy behaviour. I put this down to the fact that no one in their right mind would dare.
You know in your gut when what you're/they're doing is wrong in the relationship. Other people may have different boundaries but they're relationships have nothing to do with me. So why would it matter?