The marriage law given to the Nephites was one man and one woman—one husband and one wife. Joseph Smith taught, “I have constantly said no man shall have but one wife at a time, unless the Lord directs otherwise.” 3 Occasionally the Lord has directed otherwise, in ancient and in modern times. Monogamy has been the rule, and plural marriage the exception. The latter is to be practiced only when the Lord authorizes it, as in the case of the patriarchs Abraham, Isaac, and Jacob. The reason for the law of plural marriage is clearly stated in verse 30: to “raise up seed unto me.”
The Biblical World: An Illustrated Atlas features an article entitled “Polygamy in the Bible,” which says: “Throughout the Old Testament we read of patriarchs taking several wives… . Such polygamy was most likely motivated by the need to produce sufficient children to control the tribe’s principal asset[s]… . Childbirth was fraught with danger for both new mothers and infants. Many children were carried away by disease while still in their infancy, and many women died during childbirth. Tribes practiced polygamy as a way of sustaining themselves and ensuring the survival of the clan.” 4
In modern revelation the Lord elaborates on the reasoning behind any man’s being instructed to take more than one wife: “They are given unto him to multiply and replenish the earth, according to my commandment, and to fulfil the promise which was given by my Father before the foundation of the world, and for their exaltation in the eternal worlds, that they may bear the souls of men; for herein is the work of my Father continued, that he may be glorified” (D&C 132:63).