Talking to her about what you think of other women will not make a difference. She has already decided how she feels about herself and her body image.
Keep in mind, marriage will not help. You may be thinking 'After I marry her she will see how much I love her and have no interest in other women.' This will not work. Check out the recent study at:
http://www.buildingbetterhealth.com/...outn/103423328
People are either happy or unhappy and marriage will not change their disposition. So, you can leave or anticipate this behavior will continue indefinitely.
I married a woman with similar issues. She doesn't ask me outright what I think of other women, but I can see the hurt in her eyes if she catches me looking at another woman when we are out and about. Fortunately she has many other fine qualities that more than make up for her poor self-esteem. I just do my best to make sure she knows how sexy I think she is.