It's a very controversial rule.
But I've stood by it for many years now and I've never suffered from it.
Here's why: if they wanted to go out with you, they would ask you out! It's that simple. They don't really like being asked out because men like to compete and work hard. By not asking him out, you remain the challenge. Asking him out makes you a little too easy.
Your guy friend might say, "No way! We like being asked out!" But what they really mean is, "Of course I like being asked out! Who wouldn't want a free activity that I didn't have to plan? But yeah, if I liked you and was genuinely interested in you, I would have asked you out awhile ago." I promise you, that is what they mean. Don't believe me? Ask around. How many relationships or even second dates (where he asked her the second time because he was interested) resulted from her asking him out the first time? It's abysmally low. Much lower than the success rate when he asked her out the first time.
Some potential exceptions?
-You aren't interested in actually in productive dating (the kind of dating that leads to a relationship). You just want to test your flirting skills. By all mean, ask and practice! Just don't be disappointed when you are going on dates and nothing ever seems to come from it.
-You really, really need a date. I mean, really really really really need a date. As in, it is too awkward to go alone and it would be weird to bring a girl friend. In this situation, DO NOT ask a guy you are interested in or hope something will happen. Ask a guy friend who you've already established that nothing will happen with.
-You're a modern woman in a modern world and asking out men is now acceptable. Well, okay then. Good luck with that. Good luck going against thousands of years of evolutionary psychology. If it works and you end up marrying him, let me know. I always like being proven wrong.
My point is, don't ask guys out on dates. Flirt so that they will be comfortable enough to ask you out. But don't you dare ask him! If it seems hard at first, you'll get better at it. Trust me.