In Jacob 2, Jacob talks to the people about many of the sins they had been committing. Among those sins was the unauthorized practice of plural marriage.
23 But the word of God burdens me because of your grosser crimes. For behold, thus saith the Lord: This people begin to wax in iniquity; they understand not the scriptures, for they seek to excuse themselves in committing whoredoms, because of the things which were written concerning David, and Solomon his son.
24 Behold, David and Solomon truly had many wives and concubines, which thing was abominable before me, saith the Lord.
25 Wherefore, thus saith the Lord, I have led this people forth out of the land of Jerusalem, by the power of mine arm, that I might raise up unto me a righteous branch from the fruit of the loins of Joseph.
26 Wherefore, I the Lord God will not suffer that this people shall do like unto them of old.
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;
28 For I, the Lord God, delight in the chastity of women. And whoredoms are an abomination before me; thus saith the Lord of Hosts.
29 Wherefore, this people shall keep my commandments, saith the Lord of Hosts, or cursed be the land for their sakes.
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.
Notice that in verse 27, the Lord says that the people of that time should only have one wife. Then in verse 30, the Lord states that if He will “raise up seed unto [himself],” He will command (talking about plural marriage) His people. Otherwise, the people are to hearken to the command to have only one wife.
As part of the restoration of all things, the Lord revealed to Joseph Smith that the time had come to “raise up seed unto [the Lord].” Joseph Lee Robinson, a friend of the prophet, in his journal, recalled that in the fall of 1841, an angel of the Lord came to Joseph Smith and gave him some instruction regarding these matters.
“Joseph, unless you go to and immediately teach that principle [plural marriage] and put the same in practice, you should be slain. For thus saith the Lord, that the time has now come that I will raise up seed unto me as I spoke by my servant, Jacob, as is recorded in the Book of Mormon, therefore, I command my people.” – Joseph Lee Robinson Journal, pg 44
In the revelation given to Joseph in July 1843, the Lord once again instructed Joseph that one of the reasons for plural marriage was to “multiply and replenish the earth.” (D&C 132:63)
President Brigham Young taught this as well.
God never introduced the patriarchal order of marriage with a view to please man in his carnal desires, nor to punish females for anything which they had done; but He introduced it for the express purpose of raising up to His name a ROYAL PRIESTHOOD, a peculiar people. This revelation which God gave to Joseph, was for the express purpose of providing a channel for the organization of tabernacles, for those spirits to occupy who have been reserved to come forth in the kingdom of God, and that they might not be obliged to take tabernacles out of the Kingdom of God. – Journal of Discourses 3:264-5
A question raised by these revelations is that if God will command people to enter into plural marriage in order to “raise seed unto Him,” if we don’t live plural marriage, who are we raising seed to?