You two need to sit down and have a healthy, honest discussion. Ask her why she does that, even knowing that it makes you feel uncomfortable. Does she purposefully want to keep you off balance? Are you insecure, is that why it bothers you? Does she flirt or just say-that chick is really hot. Do you comment on actresses? or other famous people? Is there a reason for either one of you to be insecure int he relationship? Do you notice attractive people and look--just not comment?
Sorry for all the questions, I have been married for 16 years now, and yes it bothered both of us until about 11-12 years ago. We finally sat down and explained to each other that while we love and are committed to our relationship, looking and noticing people is normal. It does not mean you want to fuck them, have a relationship with them or find each other lacking. After all, even if you have a brand new car/truck--if a Ferrari, Viper or Lamborghini drove by, you would turn and stare. It does not mean you want one or do not like your new vehicle. It means you are healthy enough to be able to appreciate beauty without neglecting what you already have. Remember that you are both in a committed relationship, NOT dead!!
I hope you two work this issue out and are very happy together!! If you would like to know how we worked through our similar issue, just message me--I am always happy to help or just be there as your friend.