I gave your master's house to you, and your master's wives into your arms. I gave you all Israel and Judah. And if all this had been too little, I would have given you even more. 2 Samuel 12:8
2 Samuel. Context is everything.
Nowhere does God commend additional wives. In fact it is specifically forbidden:
First Kings: 11: 1King Solomon, however, loved many foreign women along with the daughter of Pharaoh—women of Moab, Ammon, Edom, and Sidon, as well as Hittite women.
2These women were from the nations about which the
LORD had told the Israelites, “You must not intermarry with them, for surely they will turn your hearts after their gods.” Yet Solomon clung to these women in love.
3He had seven hundred wives of royal birth and three hundred concubines—and his wives turned his heart away.
4For when Solomon grew old, his wives turned his heart after other gods, and he was not wholeheartedly devoted to the LORD his God, as his father David had been.
5Solomon followed Ashtoreth the goddess of the Sidonians and Milcoma the abomination of the Ammonites. 6So Solomon did evil in the sight of the LORD; unlike his father David, he did not follow the LORD completely.
David:
7Then Nathan said to David, “You are that man! This is what the LORD, the God of Israel, says: ‘I anointed you king over Israel, and
I delivered you from the hand of Saul. 8I gave your master’s house to you and your master’s wives into your arms. I gave you the house of Israel and Judah, and if that was not enough, I would have given you even more.
9Why then have you despised the command of the LORD by doing evil in His sight? You put Uriah the Hittite to the sword and took his wife as your own, for you have slain him with the sword of the Ammonites.
10Now, therefore, the sword will never depart from your house, because you have despised Me and have taken the wife of Uriah the Hittite to be your own.’
11This is what the LORD says: ‘I will raise up adversity against you from your own house. Before your very eyes I will take your wives and give them to another, and he will lie with them in broad daylight.
12You have acted in secret, but I will do this thing in broad daylight before all Israel.’ ”
Note that David was given the wives of Saul along with his stuff. David followed the Lord in his heart; he was quick to repent when he did wrong. But he is not commended for taking additional wives.
2 Samuel 12:
7Nathan then said to David, “You are the man! Thus says the LORD God of Israel, ‘It is I who anointed you king over Israel and
it is I who delivered you from the hand of Saul. 8‘I also gave you your master’s house and your master’s wives into your care, and I gave you the house of Israel and Judah; and if that had been too little, I would have added to you many more things like these! 9‘Why have you despised the word of the LORD by doing evil in His sight? You have struck down Uriah the Hittite with the sword, have taken his wife to be your wife, and have killed him with the sword of the sons of Ammon.
10‘Now therefore, the sword shall never depart from your house, because you have despised Me and have taken the wife of Uriah the Hittite to be your wife.’
11“Thus says the LORD, ‘Behold, I will raise up evil against you from your own household; I will even take your wives before your eyes and give
them to your companion, and he will lie with your wives in broad daylight.
12‘Indeed you did it secretly, but I will do this thing before all Israel, and under the sun.’”
13Then David said to Nathan, “I have sinned against the LORD.” And Nathan said to David, “The LORD also has taken away your sin; you shall not die.
14“However, because by this deed you have given occasion to the enemies of the LORD to blaspheme, the child also that is born to you shall surely die.”
15So Nathan went to his house.
In the 2 Samuel 12 passage you reference, God reminds David that He already gave him everything and is basically asking him why would he commit adultery with another man's wife?
If God gave David wives/concubines as a reward for his loyalty, what kind of reward would it be for him to have to take care of a lot of women he didn't know and didn't care about? Well, they already belonged to the King, and men had to take care of women in those days; they couldn't provide for themselves. If Nathan is only saying, "Hey, you had ALL these women - why Uriah's wife, Bathsheba?" then why the mention of all the stuff too? God basically said that He had done everything for David. Now the Sword will never depart from David's house, and he has to take care of all these women! Note that his sons mentioned are only the sons of his own wives.
You simply cannot use this verse and this context to state authoritatively that God approves of polygamy.