Over 90% of men and women marry, but as many as 50% of married men and women have affairs. Thus, in a self-report study of ideal mating arrangements, in which costs associated with infidelity are eliminated, it was hypothesized that men and women would prefer a mating arrangement of monogamy with the option of casual, extra-pair sex. Unexpectedly, men and women (N = 463) overwhelmingly preferred strictly monogamous mating arrangements when forced to select one such ideal arrangement. A hypothesized sex difference did emerge, however, with men more than women rating as more interesting those arrangements that included casual sex or partner variety. Discussion addresses several evolutionarily plausible accounts for the current results and integrates the current results with existing theories about sex differences and similarities in mating psychology and behavior.