Jacob 2:29-30 is where the Book of Mormon mentions the concept that Yah sometimes COMMANDS, not just allows, plural wives, when He sees need to increase the seed of His people, like in the days of Abraham and Jacob (and seems to suggest that this might also be true in the last days, or today, as we are waking up, repenting, and realizing the Law wasn't done away with, including the law of plural wives). To many people, the Book of Mormon seems to condemn the practice of plural wives, but as the above article points out, it only condemns the exploitation of it, due to the sins of the people, and David and Solomon's use of it and sexual sins with their wives and concubines (remember David's adultery with how he acquired Bathsheba, and how Solomon took many foreign wives who led him into idolatry, which Yah warned him about). Anyway, you can read the explanations in the article...
Perhaps you have a different translation of the Book of Mormon. Mine reads a little differently.
Jacob 2:23-35
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.
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.
32 And I will not suffer, saith the Lord of Hosts, that the cries of the fair daughters of this people, which I have led out of the land of Jerusalem, shall come up unto me against the men of my people, saith the Lord of Hosts.
33 For they shall not lead away captive the daughters of my people because of their tenderness, save I shall visit them with a sore curse, even unto destruction; for they shall not commit whoredoms, like unto them of old, saith the Lord of Hosts.
34 And now behold, my brethren, ye know that these commandments were given to our father, Lehi; wherefore, ye have known them before; and ye have come unto great condemnation; for ye have done these things which ye ought not to have done.
35 Behold, ye have done greater iniquities than the Lamanites, our brethren. Ye have broken the hearts of your tender wives, and lost the confidence of your children, because of your bad examples before them; and the sobbings of their hearts ascend up to God against you. And because of the strictness of the word of God, which cometh down against you, many hearts died, pierced with deep wounds.
As I understand it, permission for poly didnt come until some time later through a new revelation.