Well, we each have our limits. If she is adamant about this, it does not matter how many of us will swear that it is a wonderful experience. You can talk with her or perhaps seek counselling giving her a chance to learn how common and good it is. Otherwise, you cannot and should not force her.

lots of times women will refuse because they are nervous. They are scared that guys will find them disgusting, and it is a very exposing experience. The best way to overcome this is to assure her that you want it, that you would love how she tastes, feels, etc. Maybe she had a bad experience prior to your relationship. The best way is just to talk about it, honestly and open.