Since getting cuckolded involves some amount of surrendering the idea that you control your wife's orgasms and sexual gratification, a stranger doing her represents an extreme case of your having no control: you don't know how well he performs, your wife's attraction (one can never know precisely what things attract the wife) must be almost entirely physical. As far as I know, only one of the guys who cuckolded me was a complete stranger to my ex. But that sexual experience must have meant a great deal to her because it was the one instance of my being made a cuckold that she related to me when we were splitting up.