Well everyone has their comfort zone. The fact that she tried it says that she is speaking from experience, although it may have just been a bad experience. One thing is that I personally think a lot of girls dont' really get comfortable w/ their sexuality until they are around 30. Often their experience is based on what they "think" they should be doing, what society dictates they should do or not do and what their boyfriend tries to get them to do. This includes use of toys and some of the other more "forbidden" things such as anal.
Like the guy said - if she wants to try stomping your balls w/ lumberjack boots and you don't and she does it anyway, its probably going to be considered assault. Same if you try to "sneak it in" on her. You will be breaking a trust that is central to your relationship with her and would probably be right up there w/ rape. Sex is such a personal and intimate thing, its very tricky and requires absolute trust and respect on both sides. Do you want anal more than you want to respect her feelings? If that's true, then I wouldn't bother trying to maintain a relationship - just nail her and get a good lawyer.
Just my opinion. In time she may be more comfortable w/ the idea, but if you keep pushing it, she may feel like that's all you are interested in and that will also destroy the trust of the relationship.