actually this seems quite common nowadays where ex bfs or gfs become fubus and to those that do not know what fubu means it means f**k buddies.
But sometimes being fubus might stop you from going out with other people etc, some people kid themselves that there are no strings attached but in most cases it can be seen that either one of the partners still have feelings involved in it and are contented to be fubus in the hope of getting something more out of it in the future. This happens to both male and female as females do have sex drives too.
What I can say is this, if u want to end it with her, just stop prolonging her agony and don't give her that ray of hope that you guys can move out of the fubu stage, she deserves more than that. It will help both of you move on and find better people out there