The present study investigated the reasons why males in relationships engage in extrarelationship involvements (ERls). The theory of planned behavior was used as the basis for a decision-making model for ERI. The contribution of relationship factors and past behavior to explain ERI was also investigated. A division of perceived behavioral control into individual components of self-efficacy, control, and planning was also tested. Different sets of factors were associated with intention and behavior. ERI intentions were predicted by attitudes toward ERI, planning, game-playing love, and positive experience of past ERI. Present ERI behavior was predicted by ERI behavior in the past 6 months. Results generally supported the theory of planned behavior, especially for the prediction of intention.