Meeting the Ex

This weekend I am going to a retirement ceremony with my current boyfriend of 2 years. I was told his ex would most likely be going because she's a mutual friend of the two. I feel very uncomfortable about it and I definitely don't want to meet her. I find it wierd to meet a girl that I know he was once very serious with and actually have to talk to her and meet her. Thoughts?... we actually fought when I brought up that I was uncomfortable with it and he just couldn't see my side as to why. I'm not sure how to feel.