In Noah’s mind, the question may well have been, when Lehi commanded his sons not to have more than one wife, was that binding on all of Lehi’s posterity forever, or was it only binding on his sons? In the ancient world, fathers had the right to arrange marriages and to lay down the conditions of marriage for their sons and daughters. Any righteous person following Lehi as a prophet would certainly say that Father Lehi did not think this was a good idea, unless God should command otherwise, and thus people generally should not do this either (Jacob 2:27). However, if Noah wanted to justify himself, he could certainly have argued that Lehi was only speaking as a father binding his immediate sons, not his distant posterity.
In fact, under the Law of Moses—which Noah’s priests said they were strictly obeying—the book of Deuteronomy makes it is very clear that an Israelite man could have more than one wife. If a man was married and his brother died, he could be required under the law of levirate marriage to take his deceased brother’s wife as a second wife. The law also goes on to provide that the oldest son of a first wife was to be privileged and protected in getting a double portion in the distribution of his deceased father’s inheritance among all his sons. Thus, for example, a man could not prefer the first son of his second wife in his estate planning and give that son a double portion of the standard share of the inheritance (Deuteronomy 21:15–17).
Noah, however, was an expert at rationalizing his behavior. I am certainly not trying to justify Noah’s inappropriate conduct, but it can be instructive to imagine what reasoning he might have used to explain away his sins. When Abinadi asked what he and his priests taught the people, they did not answer that they lived Lehi’s law; rather, they replied, "We teach the Law of Moses." Noah apparently kept many of his large group of priests (likely 24 of them) busy finding ways to justify his behavior or to locate loopholes in selected parts of the written law.
Book of Mormon Central, "What Does the Book of Mormon Say About Polygamy? (Jacob 2:30)," KnoWhy 64 (March 28, 2016).