How about the unwritten command of common sense? God said that man must "cleave to his wife" and become ONE flesh. What part of one is it that you don't understand?
Here is the written command of common sense...What part of one is it that you don't understand?
Hosea
Chapter 1
1 :The word of the LORD that came unto Hosea, the son of Beeri, in the days of Uzziah, Jotham, Ahaz, and Hezekiah, kings of Judah, and in the days of Jeroboam the son of Joash, king of Israel.
2 The beginning of the word of the LORD by Hosea. And the LORD said to Hosea, Go, take unto thee a wife of whoredoms and children of whoredoms: for the land hath committed great whoredom, departing from the LORD.
Chapter 2
1 :Then said the LORD unto me, Go yet, love a woman beloved of her friend, yet an adulteress, according to the love of the LORD toward the children of Israel, who look to other gods, and love flagons of wine.
2 :So I bought her to me for fifteen pieces of silver, and for an homer of barley, and an half homer of barley:
3 :And I said unto her, Thou shalt abide for me many days; thou shalt not play the harlot, and thou shalt not be for another man: so will I also be for thee.