The D&C of Mormonism Condones Polygamy
In Mormonism, David and Solomon's polygamy was justified before God. Their wives were gifts from God and that "in none of these things did he (David) sin against me save in the case of Uriah and his wife" (D&C 132:38). Mormons have a very loose sense of the word "sin".
"David also received many wives and concubines, and also Solomon and Moses my servants, as also many others of my servants, from the beginning of creation until this time; and in nothing did they sin save in those things which they received not of me" (D&C 132:38).
"I, the Lord, ... justified David and Solomon, my servants, as touching the principle of their having many wives and concubines" (D&C 132:1).
https://www.youtube.com/watch?v=KuK1wjHION0
Mainstream Mormons Have a Proviso that in Some Cases Polygamy is Allowed by their God
Reasons for polygamy
As early as the publication of the Book of Mormon in 1830, Latter Day Saint doctrine maintained that polygamy was allowable only if it was commanded by God. The Book of Jacob condemned polygamy as adultery,[56] but left open the proviso that "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."[57] Thus, the LDS Church today teaches that plural marriage can only be practiced when specifically authorized by God. According to this view, the 1890 Manifesto and Second Manifesto rescinded God's prior authorization given to Joseph Smith.
"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" (Jacob 2:30).
However, Bruce R. McConkie controversially stated in his 1958 book, Mormon Doctrine, that God will "obviously" re-institute the practice of polygamy after the Second Coming of Jesus Christ.[58] This echoes earlier teachings by Brigham Young that the primary purpose of polygamy was to bring about the Millennium.[59] Current official church materials do not make any mention of the future re-institution of plural marriage.
https://en.wikipedia.org/wiki/Mormonism_and_polygamy