There have been many good answers given to the question posed. I would like to add a few thoughts.
When God created man, he gave man a "Free" will. With that free will man was allowed to make choices & decisions for himself, even down to the choice of serving God or not.
Had mankind not been given a free will we would have been no more than a "robot" that God could control. God did not want mankind to be mere robots that He programmed to respond to Himself. He wanted more out of the human race than that. He created man for a purpose and that purpose was to have fellowship with Him (God).
So, along with the free will that God gave to man, choices could & would be made. Some of those choices made by man were not necessarily good choices and they were displeasing to God. But God would rather to allow man to make some bad choices than to have an entity that was controlled by Himself.
Somewhere along the way some of mankind decided that it would be good for a man to have more than one wife. God allowed man to make that decision because He had given man that free will and the ability to make those choices.
Just because God allowed man to choose to have more than one wife and just because God allowed man to indeed take more than one wife does not mean that God liked it or gave His approval to it.
Take a look at the turmoil that filled many of the men's lives in the bible who had multiple wives. God may have allowed polygamy but He surely didn't approve of it, bless it or honor it. Many of these men had constant struggles in their lives that was directly related to having more than one wife.
I tell you, some of those fellas really went overboard with the free will and made some really bad decisions with the wife deal.
I'm thankful to God everyday for my wonderfully precious wife but one is enough for me or any man in my opinion!
