What is really at issue in most cases is hypocrisy. If he (or she) messes around, it is between them and their spouses (and some spouses know full well).... but if they do it while preaching fidelity and monogamy and have a history of condemning their opponents over the issue...(as several Republicans have done recently).. then it becomes a matter of 'interest' to voters who are interested in their basic character.