This way, you are in better control. It's a woman thing - go figure. Here, the assumption is, that if you answer this statement, you're assuring her that you're with her exclusively. Then, drop the subject and move on. You are specifically not telling her what she wants to hear, but you are responding to her.
Men have a bad habit of promising the world to get sex - and women know it.
Don't let her back you into a corner. To prevent her from doing this, you're going to need to establish the rules up front. She is going to assume your monogamy far sooner than you will. You don't have to always jump to answer the phone when she calls, nor do you have to return her call the same day. Get actively involved in your friends, your work, your hobbies, etc.