I told the brethren that the Book of Mormon was the most correct of any book on earth, and the keystone of our religion, and a man would get nearer to God by abiding by its precepts, than by any other book. Joseph Smith, History of the Church, 4:461
It's an abomination to God.
And now it came to pass that the people of Nephi, under the reign of the second king, began to grow hard in their hearts, and indulge themselves somewhat in wicked practices, such as like unto David of old desiring many wives and concubines, and also Solomon, his son. Jacob 1:15
Behold, David and Solomon truly had many wives and concubines, which thing was abominable before me, saith the Lord. Jacob 2:24
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 2:27
For behold, he did not keep the commandments of God, but ... he had many wives and concubines. Mosiah 11:2
Riplakish did not do that which was right in the sight of the Lord, for he did have many wives and concubines. Ether 10:5
But see also the Doctrine and Covenants 132:61-62 (also Mormon scripture), which says:
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. And 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.