The guy with the girlfriend

You know that guy? The one with the girlfriend? The one that is absolutely perfect and sweet and funny and smart and attractive? And has a girlfriend?
Well, I happen to have an amazing way with that guy. And by that I guess I mean I'm unconditionally and irrevocably, or possibly subconsciously, completely attracted to that guy. More than that, and what bothers/perplexes me the most, is that that guy is completely drawn to me. Or at least seems to be.

Somehow, though, I always think I am safe for guys. I'm the girl they have fun with in class, have wacky discussions with and take out for crazy nights of partying. But not DATE. Not that I am undatable. I'm not extremely unattractive, nor am I boring or mean, but somehow I've always been the girl that's 'one of the guys', although I walk around in 5 inch stiletto's.
And because I am completely naive, I don’t even notice these guys are flirting with me until long after I have left the club.
“You know that bartender was totally flirting with you,” my friend says just when we got out of the club and were walking home.
“No he wasn’t. He has a girlfriend. He told me so,” I naively replied.
"You're stupid. He was flirting with you,” my other friend chimed in.
Now why am I so oblivious to such behavior? Another symptom of low self-esteem? I thought I was well behind that! But my main question and annoying problem is: why am I so drawn to guys with girlfriends even before I know they have girlfriends? And most of the time after I know they do? Just as I think they don’t like me in any sexual way, am I attracted to the fact that they are safe? That they are taken so I really have no chance? And since I have no chance it’s okay to chat them up without worrying about getting hurt, or getting close, or getting to know them at all?

And to take that a little step further - I have had crushes on guys that were single, luckily. But the weird thing is, as soon as they showed interest in me and it got to a little more than some flirting, I lost all of mine. Every single time. It was like I got to my goal and the game was over. What is this? Is that my fear of commitment keeping me from getting involved with someone before it gets a little serious and protecting me from eventually getting hurt again?

Tell me, do you recognize yourself in some of this? What do you think about it?