Giving And Taking In a Relationship..

Im 22 and my gf is 20. We have been together a year in I am unhappy with our physical relationship. Not just sex but other types of intimacy. I am a very giving as far a affection goes, I constantly give her back massages, caress, and kiss her. She rarely initiates a kiss and never initiates any making out and refuses to give back massages. As far as sex goes she is my first and I gave my virginity to her because I am in love with her but sex is not what I expected it to be. It happens once every two weeks, sometimes once a week. Im often turned down when im aroused. She refuses to do any postion but misionary and lights have to be out. Sex is when she wants it, and how she wants (and she does not do any "other" sexual acts for me). She acknowledges that I am very physically affectionate and that she is not. She wants a boyfriend thats affectionate like me but feels she does not have to return the favor. She feels that it is just the way she is and if I dont like it I can leave. I feel like being affectionate toward someone you love is a choice, I dont like to give back massages any more than she does, but I like doing something that makes her feel good. Is she right, is this something I should just accept as "the way she is" or is she being a selfish lover? And why is she this way?