It's a great thing to see in movies (at least in my opinion), but I'm sure more damaging than not to a serious relationship in real life. And like others have said in related threads, once you open that door I think it's near impossible to close. Also how important would you be in your girlfriend/wife's life if she readily accepts another man sexually. I would always wonder how much my girlfriend of 3 years actually loved me if she enjoyed being with other men. I think fantasies like this should be kept to people in casual relationships or single to avoid losing something special you might have.