If this is an issue now, I would strongly discourage you from proceeding with the marriage. Sexual incompatibility is one of the leading causes of divorce. It sounds like she isn't willing to address the problem. As suggested by the earlier posting, it could be a result of trauma such as rape or abuse, a medical problem, or just a low sexual drive. However, addressing the problem requires her to be willing to do so. From the steps you have taken, I gather that she is not interested in doing so.
The question you should be asking yourself is "Am I willing to live with this for the rest of my life." My best friend ended up leaving his wife of 12 years due to this very problem. Early in relationships, it's easy to overlook problems. From your post, it is obvious that sex is a very important part of relationships for you. I think you need to seriously reconsider before taking this relationship to the next level. If two people with low sexual desire marry, there's no problem. However, if one has a much higher sexual drive than the other, this is sure to result in one being frustrated and one feeling put upon.