People tend to be a little more experimental when they are younger. She has grown and matured, and she is in a committed relationship. There would be no way I would let anyone else into the bedroom with me and my SO. It could cause a lot of problems in a relationship. And I would think anal sex would be a one try kinda thing. Give it a chance, if it isn't for you...you wouldn't want to do it again.
If you truly care about her you will respect her wishes and continue along the path to happiness.
The best to you and your relationship