I think blahblah454 had a similar problem in one of his threads
http://www.tfproject.org/tfp/showthread.php?t=110242
My advice to you is the same as it was to him. Either you accept it or you get out. Odds of her changing are just about zero.
My advice would be to cut your losses and get the hell out sooner rather than later. There are LOTS of very horny women out there who have few inhibitions and will rock your world in other ways as well.
You seem like a decent guy and like most men, we tend to be loyal to our women and breaking up with them after a length of time is usually incredibly difficult (though we as males will play bury the salami if given the opportunity). Contrast this behaviour to women however. Women will dump a guy for far less grief than you are experiencing and not give it a second thought. Far more self serving than any man. It's time to act like a woman and look out for yourself.