Girls can be, well, possessive.
Instead of looking at it in a bad way, think of it as she actually cares enough about you to let other people know you are HERS.
What that being said, as far as she is concerned, she has been the only girl in your life. She will most likely never want to know about your girl friends. If an ex is mentioned, quickly follow it with a “Stupid B—ch” and a “You are so much better than she ever was” and “I am so lucky I found you.” If she asks the “Are you going to say that about me someday?” question, here is how you answer it: “Why would I ever break up with you?” or “Never! You are so much better than any of my ex’s!”