1) Characterized by adultery 2) Extracurricular 3) Extramarital 4) Illicit 5) Like Hester Prynne 6) Unfaithful 7) Unfaithful in marriage 1) Cheating 2) Disloyal 3) Extramarital 4) Illicit 
• (a.) Guilty of, or given to, adultery; pertaining to adultery; illicit. • (a.) Characterized by adulteration; spurious.
A·dul'ter·ous adjective 1. Guilty of, or given to, adultery; pertaining to adultery; illicit. Dryden. 2. Characterized by adulteration; spurious. 'An adulterous mixture.' [ Obsolete] Smollett. 
[adj] - characterized by adultery 2. [adj] - not faithful to a spouse or lover
adulterous 1. Characterized by adultery; as, 'An adulterous relationship.' 2. Not faithful to a spouse or lover; as, 'Adulterous husbands and wives'. 3. Relating to, inclined to, or marked by adultery. 4. Characterized by adultery; 'an adulterous relationship'; 'extramarital affairs'; 'the extracurricular activities of a philandering husband'.
cheating(a) adjective not faithful to a spouse or lover; `adulterous husbands and wives`; `a two-timing boyfriend`
characterized by or involved in adultery; illicit: an adulterous relationship.