Your mind is uncontrollable, your body however is not.
Any thought that passes thru your head are no choice of yours.
Cheating is a choice, you do it with your body when you take action on the thoughts that you should have disregarded.
I class cheating as the moment you let those thoughts become actions, whatever the action is.
Ide go as far as to say that doing something as small as intentionally making sure your somewhere that the person your thinking about is just so you can see them is cheating, just alot easier to get away with than the other extremes.
If you have thoughts about someone else, then you should deliberatly steer clear of them, not encourage it by tempting yourself.
It boils down to intentions, when your questioning wether your being faithful or not, ask yourself if you are acting in the best interests or the relationship you are in, or the one that crossed your mind via lust, curiosity or whatever else might have caused it
After that, ask yourself how much your relationship means to you and youl see theres no reason to cheat either way, you leave what you had PROPERLY and pursue the new or you turn your back on your curiosity and hold strong to the one you truely care about.
(wasnt directing that at anyone by the way, well maybe past exs but they aint gonna see it 😛 )