Try freezing her out, if your not familer with this routine but you have the book "The Game" its in the book, its pretty much when you just stop in the middle of getting hot and heavy and go check your email or something...thats just a brief description but ya. So i would recomend a freeze out, and if that dosent work then tell her that..
"sex is a celebration of the intimacy two people share, and i want to share that with you"
Good line aint it?
Anyways if freeze outs, and PUA tactics dont work try talking to her about sex being apart of the relationship....or while your messing around, go down on her, eat her out. and when shese all wet and horny, then in a sly way, suggest that your going to have sex now. You might just want to shoot for "quite compliance" thats a yes in my book lol. If theres not rejection then slide in and fuck her, after that, then the cats out of the bag and you should be able to have sex whenever you want with her
If all this fails then you might want to think about another GF cuz she is one hard core Christine lol