This is the key verse for understanding why Mormons believe that Joseph Smith, Brigham Young, and other nineteenth-century Mormons were justified in their practice of polygamy, but that this is the exception to the Lord’s law, not the rule. Here, God states that if he ever does wish to institute plural marriage for the purpose of raising up seed (i.e., enabling men to have more children than they could if married to only one wife), he will make that clear by way of a specific commandment. Mormons believe that the Lord did just that in the 1830s, when Joseph Smith quietly began the practice of plural marriage. The revelation that commanded it was recorded in 1843, but the Mormons did not make their polygamous relationships public knowledge until 1852
Polygamy was officially practiced by the Latter-day Saints until 1890, when Mormon prophet Wilford Woodruff ceded to government pressure and said that the Mormons would obey the law of the land. Because some Mormons were loathe to abandon the practice and continued to secretly contract new plural marriages, the Church issued an uncompromising statement in 1904 that stipulated that any Mormon found to be practicing polygamy would be excommunicated. That has remained the Church’s policy for a hundred years; although there are some offshoot groups that still practice polygamy, members of those groups are not affiliated with The Church of Jesus Christ of Latter-day Saints. Mormons believe that the period when polygamy was publicly sanctioned (1852–1890)—and the longer period in which it was privately approved (the early 1830s to 1904)—were exceptions to God’s basic law that Jacob spelled out in verse 27.