During the reign of King Nephi II, the Nephites began to take multiple wives. Therefore Jacob said to them, "God commands me to bring you his word: 'I, the Lord Jesus, have seen the sorrow and heard the sobbing of the attractive Nephite women. You men have broken your wives' hearts, lost your children's trust, and set bad examples for them. I will not allow you to take multiple wives as David and Solomon did, for what they did is an abomination. None of you are to take more than one wife, and neither shall you take any concubines.'"
"You have committed worse wickedness than the Lamanites," said Jacob. "Even the Lamanites—who you hate for their filthiness and the curse on their skin—are more righteous than you. At least the Lamanites love their wives and children. My brothers, I fear that unless you repent of your sins the skin of the Lamanites will be whiter than yours when you stand before the throne of God."
"Therefore I give you a commandment: you shall no longer denounce the Lamanites because of the darkness of their skin, and neither shall you denounce them because of their filthiness. You will instead remember your own filthiness. Their filthiness is the result of the wickedness of their ancestors, whereas yours is the result of your own wickedness."
"Remember your children and how you've brought them sadness and set a bad example for them. Jesus says, 'I will not allow you continue to cause the attractive Nephite women to cry to me about the men. I, the Lord Jesus, delight in the chastity of women.' My brothers, awake from the slumber of death so that you do not become angels of Satan who are cast into the lake of fire and brimstone."
Jacob gave the Nephites the word of Jesus, saying, "If I, Jesus, want men to raise up many offspring to me in a short amount of time, I will command them accordingly, but otherwise you shall obey these words." Men are only authorized to take multiple wives if Jesus commands it or authorizes it through his appointed priesthood leaders. Otherwise it is a very serious sin.