I've heard many ministers respond to why God allowed polygamy by saying that it's like divorce. God allowed it but it wasn't His will. I disagree with that response. After King David sinned with Bathsheba God said to David if you had only asked "I" (God) would have given you more wives. Now, why would God not only offer but give something that wasn't his will? There are no examples in the bible of God giving someone a divorce. He would only allow it to happen in particular cases. In the case of David, it was God that offered to give him more wives. Why would He offer something that was bad? It's one thing for David to go out and get more wives and another for God to offer him more.
"Then Nathan said to David, "You are the man! Thus says the LORD God of Israel: 'I anointed you king over Israel, and I delivered you from the hand of Saul. I gave you your master's house and your master's wives into your keeping, and gave you the house of Israel and Judah. And if that had been too little, I also would have given you much more! Why have you despised the commandment of the LORD, to do evil in His sight? You have killed Uriah the Hittite with the sword; you have taken his wife to be your wife, and have killed him with the sword of the people of Ammon'"" (II Samuel 12:7-9).
God, through Nathan the prophet, told David that God had given him his position. The reference to David's master was referring to Saul. David lived in Saul's home. He had completely replaced Saul, even Saul's wife (I Samuel 14:50) and concubine (II Samuel 21:8) were now David's responsibility. Some conclude from this statement that David married Saul's wives, but this isn't a necessary conclusion and there is no mention of David having children by Saul's wives even though David's own wives are mentioned by name.
It appears that during this time, the wives of the prior king belong to the successor. That is why Abner was upset with being accused of wanting Rizpah (II Samuel 3:8) and why Adonijah asking for Abishag cost him his life (I Kings 2:23). To lay claim to the former king's wives was to lay claim to the kingship.
God said to David that He gave David everything he currently had, his wealth, power, and authority, and if these had been too little, He could have given David more. You assumed that God meant more wives, but that isn't what is stated. God could have made David the emperor of many nations. God's point is that David had it all, so his taking another man's wife was all that more despicable.
There is nothing in this passage that indicates God sanctioned polygamy.