Research exploring the determinants of infidelity has mainly focused on individual and relationship characteristics that render relationships vulnerable, paying less attention to the external circumstances that increase the likelihood of straying. The present research examined whether online exposure to norms of adultery would affect expressions of desire for alternative mates. In three studies, romantically involved participants were exposed to others’ cheating behavior and then thought of or encountered attractive strangers. Their relationship perceptions and reactions during these experiences (fantasmatic themes, expressed interest in alternative partners, and overt flirtation with them in Studies 1–3, respectively) were recorded. Results showed that following exposure to others’ cheating behavior, participants were less likely to devalue the attractiveness of alternative partners and to be committed to their relationship. These findings suggest that exposure to adultery norms decreases the awareness of long-term priorities of relationship maintenance, lessening the resistance to the temptation of attractive alternatives.