if there's nobody common in between you and her, i guess it all boils down to guts. It doesnt follow that, although very possible, if they were able to approach you...then they'd do it to others...especially when she's already in a relationship. That's another story there.
I know of someone who did approach a guy once and just once in her life coz she wanted it so bad, and maybe she thought that if she didn't do it...she would regret not even tryng. Also sis, i dont know but i think i told you about the last one, about 2mos ago, a crush of mine who actually beat me into making the first move. That was a very gutsy move, and from my perspective, risky on her part. Hey i guess she just went for the "You gotta do what you got to do" thing.
I think a lot of guys have this hidden smirk when a girl approaches them, pretty or fUgly. The turn off usually comes in, is either on how they execute it...or on how they follow through.