Polygamous "marriages" were conducted in secret by Joseph Smith as early as 1833/35 (even though the sealing power wasn't restored until 1836 and D&C 132 outlining the rules of polygamy was not given until 1843) and by a few select members beginning in 1843.
Throughout this time, the Church publicly lied that any devout leaders were involved in polygamy.
Joseph eventually "married" at least 32 women in addition to Emma Smith, including 7 girls under the age of 18 (the youngest being 14) and at least 11 women who were simultaneously married to other men.
Clearly misinformation is still taught as truth and without clear, consistent correction from the highest leadership, members perpetuate it.
Brigham Young was the most visible practitioner having been sealed to 55 women.
Sanction of the practice was supposed to officially end with the 1890 Manifesto from Church president Wilford Woodruff.
The Bible and the Book of Mormon teach that monogamy is God’s standard for marriage (emphasis added) The Lord sometimes authorizes plural marriage to have more children ("raise up seed"), as He did with Abraham, Isaac, Moses, etc.
For this reason, the Lord revealed polygamy to Joseph Smith.