[adjective] - having (had) a sexual relationship or experience with a person who is not your husband, wife or usual sexual partnerIf a man was unfaithful to me I'd leave him no matter what the circumstances.She's been unfaithful on more than one occasion.
UNFAITHFUL
Meaning of UNFAITHFUL in English
Cambridge English vocab. Кембриджский английский словарь. 2012