What is the ruling of Islamic law on falling in love with another woman, even though a man is married and has children and a beautiful wife?

A relationship with a girl angers Allah and incurs His punishment, thus immediate repentance from it is necessary. For Allah comes in between a person and his heart. It is incumbent upon whoever Allah has blessed with marriage to show gratitude for this blessing, not to meet it with disobedience. Therefore, one must cease this relationship. If the heart is attached to her, then he should propose to her and marry her, for "nothing is seen for two who love each other like marriage."