It really depends on the woman. If they are doing it in front of their man, there is your answer. She wants every other woman to dim in his eyes so he sees only her. This could be because he has made her feel insufficient or it could be because a past lover left her for a "better looking" woman.
My advise for the men is not to EVER make the person you say you love feel that they are not good enough. If you are with her, be with her. My advise to women, is if your man makes you feel ugly or unattractive, leave the jerk! find someone who will see you for the beauty you are.
Making others look bad is not part of a healthy relationship.