There have been two approaches to simulation mate-selection research. One has focused on one agent’s continuous searching in multiple periods (e.g., Mosteller, 1965; Ferguson, 1989). The other has focused on multi-agent matching in a marriage market in the same period (e.g., Gale and Shapley, 1962). The current study combines these two approaches in a new model. Using the new multi-agent continuous-searching simulation match model, this study simulates the following two mating strategies. A “one-mate dating strategy” (OMS) means dating with one mate while searching for another, better one. An “extra-dyadic dating strategy” (EDS) means secretly dating with two mates at the same time while searching for a better one. In a tenperiod limited searching situation, after 100 simulation iterations in the new model, the results show that the success rate of mating in an “OMS society” (with everyone applying OMS) is higher than in an “EDS society”. This phenomenon is called “a fallacy of composition in mate-selection”. EDS is a more unfaithful strategy than OMS, and EDS looks like a better strategy if others are applying OMS. However, in a marriage market when every individual applies EDS in mating, the results become worse. We suggest that this fallacy can be explained by the prisoner’s dilemma of game theory.