The grosser crime is that the people have begun to “excuse themselves in committing whoredoms.” The first problem with Jacob’s condemnation is one of definitions. The sin is committing whoredom, but the justification is the scriptural evidence of wives and concubines. Without yet examining the issue of wives and concubines, there is a disconnect between the definition of whoredom and either wife or concubine. The significant difference is legal. Both wives and concubines are legal, and the difference between a wife and a concubine has to do with inheritance rights for the children, not the legality of the marriage. Whoredom indicates something that is not legal.
The juxtaposition of these two suggests that Nephite law, however that was defined, did not allow for multiple wives. Therefore, any who took another wife did so contrary to law, and therefore fit the designation of whoredom.
The contradiction to current law also explains why there was an appeal to the scriptures. If the Nephites believed in the scriptures, then the scriptures could be used as a justification. That is clearly what happened, with the examples of David and Solomon being presented as men in the scriptures who legally had multiple wives and concubines.
Jacob’s response is interesting. He apparently understands that the scriptures contain history that was perhaps not all an appropriate model for current behavior. In this case, Jacob declares that Jehovah specifically “will not suffer that this people shall do like unto them of old.” This is a case where God’s current comment supersedes what was allowed at a different time.