Adultery is moral because it's biblical...
Idol worship is moral because it's biblical...
Ignoring GOD is moral because it's biblical...
Jospeh Smith was a true prophet because he ignored biblical precepts.
In at least one case, God commanded that genocide be commited. Which leads one to wonder if that makes genocide moral and justified for all time, or if it is a specific instance which does not necessarily apply to all time.
Personally, I think instances where God commanded specific people, or groups, to commit a specific act a single time does not necessarily mean that he intends for all people to commit that act anytime under any circumstances.
Considering this, even IF God had commanded a particular individual to cleave unto a woman besides his wife (which is VERY debatable), that is on a case by case basis, and does not necessarily mean ALL men should do it at all times. Context and circumstances must be taken into account. Another example would be when God’s messenger told Joseph to be accepting of Mary’s pregnancy, I do not think he meant for ALL men to overlook it when their soon-to-be bride comes up pregnant and the prospective groom knows he is not the father.