Most Helpful Girl
I've never heard that 'rule'. I don't think it's cheating. I wouldn't like it if my boyfriend flirted with other girls, but I wouldn't think he's cheating on me by doing it. What's considered cheating is something to be agreed upon between yourself and whoever you're in a relationship with. What's cheating to one couple may not be to another.
Personally, when I'm with someone, I don't really have eyes for anyone else, let alone feel like flirting with them. I'd prefer that something like that be more or less equal between myself and who I'm with. But for some people this can also be a case of, 'I don't want to, but I don't care if you do'.