Well, if you have dated her for a long time and she has become attached, it might be hard to just turn it into a friends with benefits action thing, but if its kinda a new thing you could totally keep eachother around to have fun with and f*ck. The only way to get it to work in the latter circumstance is to, through actions, convince her that you are not boyfriend material, but simply a person to have some fun with. Still, be careful, because I can tell you from personal experience and something I am going through right now, f*ck buddy things are never as simple and emotion free as they should be!