Actually the Bible NEVER called polygyny "sin" or "unclean" or anything of the sort. The Bible calls it marriage.
"He (Elkanah) had two wives. The name of the one was Hannah, and the name of the other, Peninnah. And Peninnah had children, but Hannah had no children." 1 Sam. 1:2
"David also took Ahinoam of Jezreel, and both of them became his wives." 1 Sam. 25:44
"And Joash (who was king) did what was right in the eyes of the Lord all the days of Jehoiada the priest. Jehoiada got for him two wives, and he had sons and daughters." 2 Chron. 24:2-3
Contrast that with actual adultery or homosexuality. They are clearly prohibited, rebuked, punished, and called abominations.
King David had a bunch of wives and the blessings of God were upon him. God even specifically said He gave David these wives and would have been happy to give him even more. When David took another man's wife (the Bible calls this adultery) things were a disaster and God rebuked David severely (2nd Samuel 12).
David repented of his adultery (Psalm 51), but never of his polygyny (because it isn't sexual immorality or adultery).
Regarding the laws for the king of Israel, Deuteronomy 17:17 no more prohibits polygamy than the verse right before it prohibits king from owning more than one horse.
Scripture interprets Scripture.
The Law says exactly the same thing about horses and wives "not acquire many (horses/wives) for himself".
The king was not to acquire an excessive number of horses or wives.
Are you going to argue that it is a sin for a man to own two or three horses? No one needs more than one horse. After all, a person can only ride one horse at a time.
Deuteronomy 17:17 prohibits EXCESSIVE polygamy, like that of 700/300 wife/concubine king Solomon, not the moderate polygamy of two wife king Joash or 10-20 wife king David.
"because David did what was right in the eyes of the Lord and did not turn aside from anything that he commanded him all the days of his life, except in the matter of Uriah the Hittite." 1 Kings 15:5
That verse would be nonsense if Deut. 17:17 prohibited polygyny.
David committed a crime when he took the married woman Bathsheba. He did nothing wrong when he married Abigail, Ahinoam etc.
God Himself used the language of polygamous marriage when describing His relationship with the nations of Israel and Judah. He described Himself as a Faithful Husband being married to two different (and unfaithful) women (Ezekiel 23 Oholah and Oholibah) and (Jeremiah 31:31-32 Israel and Judah).
God does not sin. Moreover, God defines what is good and evil.
God is God,.and the Bible says what the Bible says. Even when the Bible contradicts our most cherished traditions, the Christian must submit himself to God