If you flirt but know in your heart and mind that you will never do anything with this person even if he/she shows interest, then it's not cheating for me. But I think only a few - and only mostly girls - can flirt and know in their heart and mind that it's nothing. Most boys can't do that. When a boy flirts, he has an ulterior motive, and he will bite when given the chance, even when he doesn't think the girl is attractive enough. Some of them need not flirt to cheat. When a girl comes up to a boy and says "drop your pants," he will oblige. When they say dili sila mo balibad ug grasya (this one is coming from their own mouths), dili gyud na sila mo balibad ug grasya - whether or not they were the ones who initiated it.
So for me, flirting is not cheating per se, it's the motive behind it.