Some men are turned on by the fantasy of seeing their wife or girlfriend with a man who is more well-endowed or of a different race. In this case, he must convince his wife to engage in this type of activity, and it's a lot simpler if the woman feels she's in control of the situation. Other times cuckolding is used as a form of humiliation - for being a pathetic wimp who cannot satisfy his wife, he must stand and watch another man bring her pleasure.