Assume you are not exclusive unless there is a specific talk about it.
I have many female friends and FB's who have been dating a guy who thought they were exclusive, and the girl knew the guy thought so, but still considered it fair game to hook up with other guys. The girls of this mindset do not consider it cheating, because they technically never said they were exclusive (notwithstanding the fact they act bf/gf).
So, assume you are not exclusive unless the girl makes it clear that you are.
In my LTR's, I wait for the girl to bring it up and if that is what I want, I agree and then talk about cheating. It should only be talked about once, and establish your boundaries. I make it clear merely giving your phone number to another guy is cheating. She is the one who wants to be exclusive, so she can play by my rules in exchange for the exclusivity. Also, I make clear that she will be dumped if she ever cheats, and I will cut all contact with her and never speak to her again if that happens. That kills any dream of getting back together after a breakup.