This study investigated how men and women perceive online and offline sexual and emotional infidelity. Undergraduates from a large university in Northern Ireland participated in the study. It was found that men, when forced to decide, were more upset by sexual infidelity and women by emotional infidelity. It was also found that men were more likely to believe that women have sex when in love and that women believe that men have sex even when they are not in love. It was not, however, found that either men or women believed that having cybersex implied the other was also in love or that being in love online implied they were having cybersex. These results are explained through a social-cognitive lens.