Isn't it wrong for my husband to stay in touch with high school girlfriends?

My husband has already cheated on me twice. We have been married for 16 years and he swears he will not cheat again, but he stays in touch with old high school girlfriends on facebook and will sometimes go to lunch with them when they are in town. What is his problem? I don't make a big deal out of his friendships but it is hurtful to me. What he does seems to give him a high or something. After sixteen years, I really have not changed. I try to stay in shape, look my best and be a good wife. What is wrong with him?