is a single catholic woman engaged in an affair with a married man guilty of the sin of adultery?
The answer is yes, even though no overt sexual acts have taken place. St. Thomas Aquinas identifies adultery primarily as a sin against justice. It is the taking of what rightly belongs to another (time, affection, intimacy and especially, exclusivity). I alway counsel persons who have engaged in this sin to pray for the innocent spouse. It is a great sorrow to be betrayed in this way.
Fr. Vincent Serpa, O.P.