Look deep within. For example, you said to Mathew "You don't even know me" .... but if you apply that attitude towards all the strangers who chase after you, where does that get you?
Maybe it's less about being open than about taking risks. If you shut down a guy before he even gets a chance to show his personality then you've gained nothing and lost nothing.
If you give him a chance, you can always shut him down later (losing nothing), and maybe find someone who matches your style.
That way you don't need to really be "open" and accept people you wouldn't normally like. That's just called lowering your standards, and why should you have to do that?