WordNet sense 1 (characterized by adultery):

extracurricular, extramarital

WordNet sense 2 (contrary to accepted morality (especially sexual morality) or convention):

illicit

WordNet sense 3 (not faithful to a spouse or lover):

cheating, two-timing

WordNet sense 4 (having sexual relations with someone other than your husband or wife, or your boyfriend or girlfriend):

unfaithful