Don't screw with the wildlife, she wants a relationship, if you wont give her that, let her be happy with some other guy, instead of miserable stroking your shallow ego. I'm sorry but I spend enough time pissed at women for doing this to men, that it's only fair that I be open and honest.
With my pets, which she is sounding like, as soon as she finds a good guy that she wants, I tell her "Cutie, go with him, there is no future with me, he will take care of you", I have one women who I possess, and she gets all the commitment she needs from me, the rest of the women, I am ready to give them away as soon as they are looking for something more. No games played.
She wants something different than you, don't make her miserable by playing with her, and don't make yourself miserable by doing something you don't want. And then at least she can be honest, and dump this looser she is with and stop making him miserable as well, lol!
edit: And for the young'ns reading this, classic college freshman line "I don't trust myself alone with you" means " I am going to fark you when we are alone, (but I dont want to feel like a slut, unless she does want to, lol! ) "