Drawing on ethnographic and interview data collected from the United States and Finland on lifestyle ("swinging") events, this article explores the implicit and explicit rules influencing negotiations for group sex as a type of play. Participants maintain a sense of freedom and spontaneity while acting within situational constraints—ethical expectations, preexplicated rules, implicit rules, and complex negotiations that occur during the play itself either openly or more subtly. Because it has implications for the participants’ everyday lives, lifestyle group sex is a phenomenon on the border between games and adult play. Through an analysis of the rules and social contracts arising in group sex, we demonstrate how participants learn to read interactions at group sex events in the way that players learn game systems and how they can and do become "good players" in such situations.