Why do woman that have boyfriends avoid their guy friends?

This questions for the ladys. Why is it that when woman have boyfriends always try to avoid their guy friends? Is there a difference whether the gu friend is attractive or not? Are they afraid their boyfriends ill get mad? I've noticed that when I tex a buddy who has a boyfriend, she trys to avoid me and when she doesn't, persues me. Hmm iono.