God’s reason for giving this command is because of how important marriage is in the heart of God. God created marriage between one man and one woman and was the building block of His creation and of society. Even after the fall marriage is a sacred union in the way the man and woman complement one another and bring new life into the world through their marriage. With such a premium placed on marriage, it is no wonder that God would seek to protect this union from being defiled which is what adultery is and that is really the Heart of the Matter.