Concubines in the Old Testament “were considered to be secondary wives, that is, wives who did not have the same standing in the caste system then prevailing as did those wives who were not called concubines” (Bruce R. McConkie, Mormon Doctrine, p. 154). Concubines had full protection as wives and did not violate the law of chastity when the marriages were approved by the Lord (see D&C 132:34-43). In the time of King Noah, however, the word “concubines” (Mosiah 11:2) referred to the wicked practice of a man living with more than one woman, in or out of marriage, without God’s approval.
The word “whoredoms” (Mosiah 11:2) refers to any perversion of the laws of chastity and virtue. [Book of Mormon Student Manual for Religion 121 and 122, p. 62]