Ten Commandments
To "be false to the married relation", called adultery, is when a married person has sexual relations with a person other than his or her spouse. Having sex outside of  marriage is fornication and is also sin. It is condemned in other places in the Bible, but not specifically in the Ten Commandments.  Jesus taught his audience that the outward act of adultery does not happen apart from sins of the heart: "From within people, from their hearts, come evil thoughts, unchastity, theft, murder, adultery, greed, malice, deceit, licentiousness, envy, blasphemy, arrogance, folly. All these evils come from within and they defile.” In The New Testament Jesus says "But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart."