It is hard to imagine how Orihah had 31 children unless he had more than one wife. The practice of polygamy (or more accurately, polygyny) was probably regularly practiced among the Jaredites. The brother of Jared had 22 children (Ether 6:20), Shule also begat many sons and daughters (v. 12), Corihor had many sons and daughters (v. 14), and the people had become exceedingly numerous (v. 11) in the space of only a couple of generations. The practice was abused by Riplakish, who did not that which was right in the sight of the Lord, for he did have many wives and concubines (Ether 10:5). The record implies not that he started the practice, but that he abused it.
However, among the Lamanites and Nephites, the practice of polygamy was expressly forbidden, For there shall not any man among you have save it be one wife; and concubines he shall have none (Jacob 2:27). The only known instance in which it was practiced among them was the wicked king Noah, who had many wives and concubines…and they did commit whoredoms and all manner of wickedness (Mosiah 11:2).
Thus, the Lord allows the practice under certain special conditions—specifically, he has said, if I will…raise up seed unto me, I will command my people; otherwise they shall hearken unto these things (Jacob 2:30).