my advice...
ive been there.. girl really into you and wants to keep fucking you and you *really* like her but for whatever reason won't commit.. keeping herself for somebody (ex) shes in love with.. closet lesbian.. has an arranged marriage.. doesnt really matter the reason...
here's the deal.. you really have two options.. push the lock down or enjoy the ride..
pushing the lockdown:
-- being honest with yourself, reason you want the lockdown is to keep her from the other guy.. before the whole her denying the bf thing you were fine with fun and plenty of sex and letting the relationship thing develop as it will
-- so here it is.. if you push the relationship, you lose the sex. she will eventually cut you off if she feels pressured for the relationship...
-- direct honestly, she has a reason shes not in a relat with you though she obviously likes you.. and theres probably nothing you can do about it...
-- on a positive note, you will then know where you stand and not risk getting yourself hurt.. you stand up, be strong, and have closure one way or another
-- this is the method i took.... I REGRETTED IT!!
hanging back/ being the fuck buddy:
-- trust me, if you push the relationship and lose the sex, you will regret it.. when it dries up you wont care if she doesnt date you as long as she fucks you..
-- so you need to lay back.. (i have not done this correctly yet, because its *very* hard, but heres how to do it)
1) like you have been doing, do *not* mention a relationship.. if she brings it up, just say "we're having a lot of fun and you didnt want a relationship for whatever reason, so when you're ready to talk about what's holding you back, we'll talk.. meantime, come over here..

" - you get the idea..
2) and this is probably the most important!! you have to get hold of your mind!!
a. she has somebody else, so she will *never* get one-itis for you
b. you need to make sure you take 2-3 days a week to set up someone else on the side to help fight your own inevitable one-itis. once you have one or two more girls like her in the wings, having her as a fuck buddy will be just an awesome bonus and you may even lock down one of the other girls...
c. KEEP FUCKING HER - as long as you are fucking her, she also cannot commit emotionally to somebody else and she will get more attached.
d. tell her if you cannot see her because of the other girl (I guarantee she is seeing someone else, so she will not blame you for this, quite the opposite, it will trigger jealousy/ loss), just dont make it crass.. simple "i have plans that night" is subtle enough..
e. MOST IMPORTANT AND THE HARDEST!!! get control of your mind...
-- eventually the other guy will find out and if you are being cool, when he freaks, you will suddenly have more value than him
-- if shes with you, you have something he doesnt so he is really no threat
-- take WHATEVER time you need to maintain your cool. if you have to avoid her for a couple days/ weeks to be cool, do so.. just never tell her why.. let her think you're with another girl..
-- releas your inevitable anger and frustration through sex... its the only way that will work... be fun at all other times..
-- the way to win is not to compete.. get the frame that you are the best choice for her and if she makes the wrong choice you will be sad for her for making the wrong choice..
Trust me dude.. although this is by far the hardest (if not almost inhumanly possible) road, you will regret pushing the relationship... better to enjoy her as an awesome fuck buddy and control your mind/heart/ and situation to help you find another one.. (you can even use her as social proof to help you find her replacement - girls judge the quality of the girl you are with, so she will attract girls of like quality)
theres always the possibility she may change her mind or whatever situation is holding her back will change, but if you're still fucking her when it happens, you're in there. if you push her away, you may not be next in line when it happens..