In verse 27 Jacob declares, "hear me, and hearken to the word of the Lord." So this is apparently a law that they have not had before. Now, was it permissible or not under the Law of Moses to have more than one wife? It was permissible. Deuteronomy 21:15–17 says that a man cannot prefer the first son of his second wife over the first son of the first wife, so we know that there were plural marriages. Deuteronomy 17 says that the king, who may well have several wives, should not multiply wives or gold unto himself. So the problem was with excess, going to an extreme, having too much or too many. Deuteronomy 17 says that even the king cannot go too far.
Now what I think we may have going on here is a little bit of tension between the temple and the palace. It may have been the kings who followed Nephi who were modeling their conduct after David and Solomon: "for they seek to excuse themselves in committing whoredoms, because of the things which were written concerning David, and Solomon his son. Behold, David and Solomon truly had many wives and concubines" (Jacob 2:23–24). But Jacob, speaking on behalf of the Lord, tells them that such practices were "abominable" (v. 24). Why? Well, they had way too many wives, especially Solomon who had a thousand wives, most of whom were foreign women. In verse 26 Jacob proclaims, "Wherefore I the Lord will not suffer that this people shall do like unto them of old."
So this is apparently a new law that Jacob is giving them. And what is the law? It is that "there shall not any man among you have save it be one wife; and concubines he shall have none" (Jacob 2:27). Concubines were secondary wives (not prostitutes, as some might assume). They just did not have inheritance rights and so on. But Jacob is saying that the people can’t engage in such practices at all. He explains why in the next verses: "For I, the Lord God, delight in the chastity of women. And whoredoms are an abomination before me; thus saith the Lord of Hosts. Wherefore, this people shall keep my commandments, saith the Lord of Hosts, or cursed be the land for their sakes" (vv. 28–29).
Then, in the next verse, the Lord clarifies the matter even further: "For if I will, saith the Lord of Hosts, raise up seed unto me, I will command my people; otherwise they shall hearken unto these things" (Jacob 2:30). So there is an exception to this law. Polygamy can be permitted, but only when the Lord commands for the purpose of raising up a righteous people. That is what happened with Abraham, Isaac, and Jacob in order to fulfill the promises that they would have posterity that would be able to bless the entire earth. But Lehi’s family was not in that situation, in several ways.
Under ancient laws, fathers normally negotiated the prenuptial agreement with the bride for their sons. A lot of these marriage contracts are documented, and usually they contain a provision where the father of the bride has control over whether or not the groom can take a second wife or not. And if the bride’s father doesn’t want this to happen, we will see something in the contract that says in effect, "This marriage can go forward, but my daughter will not have a second wife in the home to deal with." On the other hand, sometimes the father of the bride didn’t care. He may have thought, "Well sure, that might be a good thing. My daughter will be the primary wife, and if there is a second wife then my daughter will have someone to help her with the cooking and the household work and so on." Maybe it depended on what kind of a bargain the father could drive or what kind of a dowry the groom or father of the groom was offering in the deal. These things were negotiated.
There may well have been some kind of an agreement between Lehi and Ishmael when Ishmael’s daughters married Lehi’s sons. Notice that it says here that Lehi was the one who prohibited his sons to engage in plural marriage (Jacob 2:34). That was a right that he as a father had over his family. What I see going on here with Jacob is taking this matter one step further. Not only was this what their father Lehi did for their generation, but now this is the rule of the Lord for this people as a whole. Apparently, as they negotiated the terms of these marriage contracts, some of the fathers were saying, "Well, polygamous relationships would be okay with me." And Jacob is saying, "No, that’s not going to work because it is against the commandments of the Lord for our people."
Also, sometimes there were conditions in these marriage documents about children as well. Not only could a second wife be taken if the first wife did not produce children, but sterility or infertility was, in the ancient world, an automatic ground for divorce. We see Abraham in that situation, where someone in Abraham’s situation normally would have gotten rid of Sarah long ago, because he had been promised that he would have all these children and it wasn’t working. But Abraham did not do that, no doubt realizing the eternal nature of his covenants and God’s promises.
Book of Mormon Central, "What Does the Book of Mormon Say about Polygamy? (Jacob 2:30)," KnoWhy 64 (March 28, 2016).