This is what the D&C says about polygamy:

16 Wherefore, it is lawful that he should have ONE WIFE, and they twain shall be ONE FLESH, and all this that the earth might answer the end of its creation; (D&C 49, 1831)

This is what the Book of Mormon says about polygamy:

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. (Jacob 2)

The greater context reads thus:

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.
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. (Jacob 2)

Note, God is mindful of how polygamy affects children, the daughters. He is mindful that tender feelings are tied to chastity. Only in very rare circumstances will God "command" its practice in order to "raise up seed." Joseph did not raise up any seed by any of his polygamous marriages. Indeed, he married many women who were:

  1. Already married
  2. Already married to faithful Mormon men
  3. Already sealed to their faithful Mormon husbands
  4. Already able to have children by their faithful Mormon husbands
  5. Already had children by their faithful Mormon husbands

This is what the Bible say about polygamy:

  • Bishops: the husband of one wife -1 Tim. 3:2
  • Elders: the husband of one wife -Titus 1:6
  • Deacons: the husband of one wife -1 Tim. 3:12

The Mormon Church has acknowledged that Joseph’s polygamous marriages were sexual in nature and to already married women (polyandry):

For a Church-approved work to state that polygamy involved “conjugal relations” and use the term polyandry is landmark. (The Joseph Smith Papers, Journals, Volume 2: December 1841–April 1843)

Here is a comment by one of the sons of Joseph Smith:

"April 24, 1863, President Smith wrote a letter to Elder Charles Derry, first foreign missionary of the Reorganized Church, then in England. This is a mingling of public and private sentiment, but of interest in studying the character of the man. It is as follow:"

"There is a crisis approaching in Utah affairs. Brigham and Wells and Taylor are under bonds to answer in the United States courts for polygamy contrary to the statute prohibiting the practice in the territories." (Herman C. Smith, ed., Journal of History, RLDS, vol. 12, no. 1, Jan. 1919, p. 7)

"Happy am I (me thinks) who have never seen the full and fatal effects of the doctrine you and I so much deprecate, except under the influence of what mellowing features time and distance intervene. Nevertheless, enough has been refracted through intellectual sources to create and foster a complete and never to be eradicated loathing of its defenders and propagators, and intense hatred of the thing itself. My earliest inclinations were against it; and the testimonies of the gospel have clasped and manacled the hideousness of its evil upon my heart of hearts. And may God so order it that you and I may long stand as shoulder to shoulder in the great fight of light and truth as against darkness and error, to the final overthrow of the doctrine of polygamy and all its adjuncts, amen." (p. 8)

In everything there must be two or more witnesses of which there are none for the type of polygamy Joseph Smith practiced.