How the church began
Plural marriage
Joseph Smith married many women, including some already married to other men and one who was 14. The Church's own essay says so.
In the Church's words
Section 132 of the Doctrine and Covenants, recorded in 1843, presents plural marriage as commanded by God. The Church's essay acknowledges Joseph Smith's many marriages, including to women already married and to a girl of 14, and says his wife Emma found it an excruciating ordeal. The Church announced the end of new plural marriages in the 1890 Manifesto, now Official Declaration 1.
For Joseph Smith’s wife Emma, it was an excruciating ordeal.
The Church's own words Gospel Topics essay, "Plural Marriage in Kirtland and Nauvoo" · source
61And again, as pertaining to the law of the priesthood—if any man espouse a virgin, and desire to espouse another, and the first give her consent, and if he espouse the second, and they are virgins, and have vowed to no other man, then is he justified; he cannot commit adultery for they are given unto him; for he cannot commit adultery with that that belongeth unto him and to no one else. 62And if he have ten virgins given unto him by this law, he cannot commit adultery, for they belong to him, and they are given unto him; therefore is he justified.
Doctrine and Covenants 132:61-62 · read it at ChurchofJesusChrist.org
What the Bible says
Jesus took marriage back to the beginning: one man and one woman, the two becoming one flesh. Polygamy appears in the Old Testament as something that happened, and its stories are stories of rivalry and grief; it is never God's design. An elder is to be the husband of one wife.
4He answered, “Haven’t you read that he who made them from the beginning made them male and female, 5and said, ‘For this cause a man shall leave his father and mother, and shall be joined to his wife; and the two shall become one flesh’? 6So that they are no more two, but one flesh. What therefore God has joined together, don’t let man tear apart.”
Matthew 19:4-6 (WEB) · context
2The overseer therefore must be without reproach, the husband of one wife, temperate, sensible, modest, hospitable, good at teaching;
1 Timothy 3:2 (WEB) · context
Sources
- Gospel Topics essay "Plural Marriage in Kirtland and Nauvoo"; Doctrine and Covenants 132; Official Declaration 1 (churchofjesuschrist.org).