In both these scriptures the Lord gives two situations where plural marriage is appropriate. In both instances, when the injustice of men is considered, and the practice is condemned.
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And if you fear that you will not deal justly with the orphan girls, then marry those that please you of [other] women, two or three or four. But if you fear that you will not be just, then [marry only] one or those your right hand possesses. That is more suitable that you may not incline [to injustice]. (http://quran.com/4)
The Book of Mormon: Jacob Chapter 2:27 Wherefore, my brethren, hear me, and hearken to the word of the Lord: For there shall not any man among you have save it be one wife; and concubines he shall have none;
30 For if I will, saith the Lord of Hosts, raise up seed unto me, I will command my people; otherwise they shall hearken unto these things.
31 For behold, I, the Lord, have seen the sorrow, and heard the mourning of the daughters of my people in the land of Jerusalem, yea, and in all the lands of my people, because of the wickedness and abominations of their husbands. (Italics added)
Here is an exerpt from the LDS.org article on Plural marriage.
According to Helen Mar Kimball, Joseph Smith stated that “the practice of this principle would be the hardest trial the Saints would ever have to test their faith.” Though it was one of the “severest” trials of her life, she testified that it had also been “one of the greatest blessings.”46 Her father, Heber C. Kimball, agreed. “I never felt more sorrowful,” he said of the moment he learned of plural marriage in 1841. “I wept days. … I had a good wife. I was satisfied.”47
The decision to accept such a wrenching trial usually came only after earnest prayer and intense soul-searching. Brigham Young said that, upon learning of plural marriage, “it was the first time in my life that I had desired the grave.”48 “I had to pray unceasingly,” he said, “and I had to exercise faith and the Lord revealed to me the truth of it and that satisfied me.”49 Heber C. Kimball found comfort only after his wife Vilate had a visionary experience attesting to the rightness of plural marriage. “She told me,” Vilate’s daughter later recalled, “she never saw so happy a man as father was when she described the vision and told him she was satisfied and knew it was from God.”50 (https://www.lds.org/topics/plural-marriage-in-kirtland-and-nauvoo?lang=eng)
