For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
Polygyny is not condemned in the Bible, wheras polyandry is considered adultery. Bigamy (so long as it is adding a wife), is also not condemned as a crime in the Bible and is actually preferred to divorce. [It's still illegal in America, though].
Just my thought,... Just as Solomon, and the many other Kings that had many wives and concubines, this was not their sin, nor a hardness of their hearts (because they were favored by God) but that the women, foreign women, lead them to worship Pagon Gods. This was their separation from God, not the wives and concubines in themselves.
Solomon did err in that he 'multiplied wives', he also 'multiplied' horses and gold. These were all things that a king was to be on guard against (Deut 17:15-17), as accumulating excess leads to the trouble of the heart turning away from solely serving God.
There was a large difference between the multiplication of wives that Solomon did, and the regular 'adding' of wives that was common in the time. David/Abraham, etc, added wives, Solomon multiplied them beyond all reason (I Kings 11:1-3)
Here's a verse about a different king that did what was right in the sight of the Lord, and was given two wives by Jehoiada the priest:
"Joash did what was right in the sight of the LORD all the days of Jehoiada the priest. Jehoiada took two wives for him, and he became the father of sons and daughters" II Chron 24:2-3
Why does anyone discuss the righteousness of God and compare it with the laws of America? Whether polygamy was "a sin" is of no real importance for us to figure. Was it of God? I challenge anyone to point out a place in scripture where God instructed a married man, a husband, to go and get himself another wife.
God has a purpose for all of us. For the most part, He's quiet about what that purpose is.
Jacob's father-in-law, Laban conspired with his two daughters and got Jacob to marry both of them (Genesis 29). God had nothing to do with that confusion. But He had a purpose for staying quiet while the conspiracy played out. Jacob ended up having sons by both wives and both of their hand maidens.
God doesn't endorse trickery of any kind; but it won't stop Him from carrying out His purpose for us.
He didn't keep quiet when Sarai, Abraham's wife was taken by the king of Egypt. But He did keep quiet when Abraham and Sarah came up with the idea of having a son by mating Abraham with Sarah's hand maid. In both instances, He acted according to His purpose.
God isn't the captain of the divine police department, keeping law and order and trying to stamp out sin. He's the God of our salvation. He saves polygamists from all of their sins. What difference does it make whether polygamy is "a sin" or not? They were sinners in need of salvation the day they were born, before they ever did anything good or bad..