when one is young one,s self esteem is generally low so your partner enjoying sex with a third can be a belittling experience, As one gets older self image is not so important and watching your wife enjoying another body is quite exhilarating and a big turn on. You do not feel slighted because you know "you are the man" and your wife,s happiness is a priority. A further thought, is it okay for the wife to suggest it or is it only ok if the hubby suggests it. I don't mind but a little bit of something different is exciting. Men love women who love sex.