Adultery
Adultery is unnatural relations between a married person and a person other than their spouse, as described in The Catholic Encyclopedia [1] The Bible forbids adultery in the Ten Commandments (Exodus 20:14).
Jesus said that a man commits adultery in his heart by lustfully looking at a woman. (Matthew 5:28) Sexual intercourse by a married man or woman outside marriage is considered adultery under modern law, although it is hardly ever punished anymore.
Adultery ruins trust between husband and wife and destroys families. It is contrary to the traditional marriage pronouncement derived from Matthew 19:6: "Wherefore they are no more twain, but one flesh. What therefore God hath joined together, let not man put asunder." It also breaks the promise that the bride and groom make to each other in virtually every form of marriage ceremony observed in the U.S.—even secular observances conducted in judge's chambers, or in vows written by the couple themselves.
Notes and references
http://www.newadvent.org/cathen/01163a.htm