"And Sarai said unto Abram, Behold now, the Lord hath restrained me from bearing: I pray thee, go in unto my maid; it may be that I may obtain children by her. And Abram hearkened to the voice of Sari.
And Sarai Abram's wife took Hagar her maid the Egyptian, after Abram had dwelt ten years in the land of Canaan, and gave her to her husband Abram to be his wife."
I had heard the practice of plural marriage was called the "law of Sarah". I did a little scripture chase to find the reference to it.
Doctrine & Covenants 132: 34, 65
"God commanded Abraham, and Sarah gave Hagar to Abraham to wife. And why did she do it? Because this was the law; and from Hagar sprang many people. This, therefore, was fulfilling, among other things, the promises.
Therefore, it shall be lawful in me, if she receive not this law, for him to receive all things whatsoever I, the Lord his God, will give unto him, because she did not believe and administer unto him according to my word; and she then becomes the transgressor; and he is exempt from the law of Sarah, who administered unto Abraham according to the law when I command Abraham to take Hagar to wife."
I am grateful that when the "law of Sarah" had to be restored in the fullness of times it was again by the first wife's consent. I am also glad that the law has been fulfilled and no longer practiced. To practice plural marriage outside of the Lord's command would be a perversion. Such things are only revealed through God's prophet.