How do we respond to unfaithfulness in marriage?

Matthew 5:27-32“You have heard the commandment that says, ‘You must not commit adultery.’ But I say, anyone who even looks at a woman with lust has already committed adultery with her in his heart. So if your eye—even your good eye—causes you to lust, gouge it out and throw it away. It is better for you to lose one part of your body than for your whole body to be thrown into hell. And if your hand—even your stronger hand—causes you to sin, cut it off and throw it away. It is better for you to lose one part of your body than for your whole body to be thrown into hell. You have heard the law that says, ‘A man can divorce his wife by merely giving her a written notice of divorce.’ But I say that a man who divorces his wife, unless she has been unfaithful, causes her to commit adultery. And anyone who marries a divorced woman also commits adultery.”

Note that Jesus pairs his teaching on lust directed toward men with his teaching on divorce also directed toward men, citing unfaithfulness as the only legitimate justification for divorce. The warning about lust as the equivalent of adultery creates a compelling caution in a society where men typically could divorce their wives for any reason. By Jesus’ definition, few married men could claim they had never been unfaithful.