Rodney Turner writes that in condemning the taking of "many wives and concubines," Jacob did not proclaim a new doctrine. He told the Nephites: "Ye know that these commandments were given to our father, Lehi; wherefore, ye have known them before" (Jacob 2:34; see also 3:5). Nephi records that his father wrote "many things which he prophesied and spake unto his children" of which Nephi did not make a full account on the small plates (see 1 Nephi 1:16). Thus, although the law of Moses permitted wives and concubines, the Lord apparently forbade the practice for the house of Joseph in the Promised Land, in the Americas. This was probably in part because of its historic abuses, but also because the basis for such marriages did not exist in Lehi's colony.
Concubines were not mistresses or prostitutes, they were lawful wives---usually captive slaves or foreigners--who had legitimacy but not full honor. Their children enjoyed no rights of inheritance. It was a case of social inferiors becoming part of a man's family. Concubinage reflected the realities of the ancient world. It was a lesser law for a lesser time. In viewing those times the issue is not what was ideally right or wrong, fair or unfair, but what was workable. If concubinage was a relative evil, it was the lesser of evils; better a concubine than a woman alone, or a harlot. That the Lord justified his servants in having concubines--and he did--is no proof that he viewed the practice as more than a necessary, albeit unfortunate aspect of an imperfect order of things.
The Nephites did not practice slavery, nor did they take female captives and make wives of some of them as had their Israelitish ancestors even in the days of Moses. As for the many war-produced widows found at times among the Nephites, the policy was to care for their temporal needs rather than to marry them (see Mosiah 21:10, 17; Moroni 9:16). [Rodney Turner, "Morality and Marriage in the Book of Mormon," in The Book of Mormon: Jacob through Words of Mormon, To Learn with Joy, pp. 280-282]