I had a very aloof gf who was cool to sex like that. One of the other suggestions above, to refuse her sex next time she wants it (or goes along with it), is the right one. This gf, later, after we broke up, told me about her bf that she was with after me. (Well, I asked a lot of questions about him and how she was with him.) She said he was incredibly thick, that she was "shocked" when she saw how big around. When I asked if it helped her to be more sexual than she had been with me, she said he refused to have sex with her until she wanted it so much she begged for it.
I said something along the lines of "He must have been disappointed" or whatever, implying he would have to wait til Hell froze over before she would beg for sex.
She replied, "No--I did."
"Huh?" I responded.
"I did beg for it," she admitted...
This is EXACTLY what you should do. And let us know how it works.