Legs come and go, what if she becomes a paraplegic?!

It's only a phase (in regards to your leg obsession), man. Like you said, those legs aren't going to be in that condition forever and unless you see other meaningful things in a relationship with her, you probably shouldn't be pursuing it especially if she has a boyfriend (I guess you could always wait for an unlikely breakup, though).

If you have low self-esteem about being a loser in the dating crowd, look around and get some practice! Who knows, you might hit the jackpot with "amazing legs"!