More accurately, in the early part of the OT polygamy was just taken for granted.
While the Law may not have sanctioned it, I am also unaware of anywhere it was prohibited. In fact, it has sections whereby it is regulated.
Many of God’s favorite people were polygamists: Abraham, Jacob, Judah, David, Solomon, etc. He did not cut them off from his favor for this.
As I said above, it was just taken for granted as a part of life. Not encouraged, but also not prohibited, and not an inkling given that it was sinful.
In fact, the Bible doesn’t specifically prohibit polygamy in the New Testament either.
Many of God’s favorite people were polygamists: Abraham, Jacob, Judah, David, Solomon, etc. He did not cut them off from his favor for this."
All we need to do is look at God's original design for marriage, which was one woman for one man. In Genesis, He said He would make a helpmeet--singular---for Adam. Not a couple or a few of them.
The Bible says that God hates divorce, yet sinful human beings disobeyed His intentions there, as well. So He provided guidelines to be used in the event of divorce, knowing that man would continue to distort His will for marriage.
Deuteronomy 17:17 specifically forbids Israel's kings to take multiple wives. Also, polygamy is addressed in Malachi 2.
As for the men you call God's favorite people, they were all sinners, like the rest of us, and some of their sins were described in the Bible. David was an adulterer and a murderer, but neither was sanctioned by God. His grace and love is given in spite of our sins.
Divorce, as an institution, Jesus told us frankly that God tolerated it because "your hearts were hard." It was certainly not his plan, and marrying after divorcing a wife make adulterers of all concern.
If you are an adulterer after divorcing a wife and marrying another, you are clearly an adulterer if you marry more than one at the same time. Deacons were supposed to be a man who had only one wife, so clearly monogamy was a value. It just was not a value men could respect when they wanted to abandon the first wife and find one younger.
You shouldn't take as a moral example David, Abraham, etc because while they were loved and found favor, were all capable of the same sin we are all capable of and their examples of failure make up most of the stories in the OT. Sarah's promotion of Hagar was in disobedience to prophesy of angels--and we're still paying the price with Ishmael's descendants who are as numerous as the grains of sand in the desert.
1 Timothy 3:2, 12 "A bishop then must be blameless, the husband of one wife, vigilant, sober, of good behaviour, given to hospitality, apt to teach. . . . Let the deacons be the husbands of one wife, ruling their children and their own houses well."
God ordained one man and one woman in the beginning.
Genesis 2: 20 And Adam gave names to all cattle, and to the fowl of the air, and to every beast of the field; but for Adam there was not found an help meet for him. 21 And the Lord God caused a deep sleep to fall upon Adam, and he slept: and he took one of his ribs, and closed up the flesh instead thereof; 22 And the rib, which the Lord God had taken from man, made he a woman, and brought her unto the man. 23 And Adam said, This is now bone of my bones, and flesh of my flesh: she shall be called Woman, because she was taken out of Man. 24 Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh.
Did you notice it didn’t say “wives”?
Polygamy started with the offspring of the first murderer. Those that were rebellious to God.
Genesis. 4: 16 And Cain went out from the presence of the Lord, and dwelt in the land of Nod, on the east of Eden. 17 And Cain knew his wife; and she conceived, and bare Enoch: and he builded a city, and called the name of the city, after the name of his son, Enoch. 18 And unto Enoch was born Irad: and Irad begat Mehujael: and Mehujael begat Methusael: and Methusael begat Lamech. 19 And Lamech took unto him two wives: the name of the one was Adah, and the name of the other Zillah.
God forbid kings to multiply wives. Deut. 17:14 When thou art come unto the land which the Lord thy God giveth thee, and shalt possess it, and shalt dwell therein, and shalt say, I will set a king over me, like as all the nations that are about me; 15 Thou shalt in any wise set him king over thee, whom the Lord thy God shall choose: one from among thy brethren shalt thou set king over thee: thou mayest not set a stranger over thee, which is not thy brother. 16 But he shall not multiply horses to himself, nor cause the people to return to Egypt, to the end that he should multiply horses: forasmuch as the Lord hath said unto you, Ye shall henceforth return no more that way. 17 Neither shall he multiply wives to himself, that his heart turn not away: neither shall he greatly multiply to himself silver and gold.
Most men of God even in the Old Testament had only one wife. This is true for Adam (Gen. 2-4), Noah (Gen. 6:18), Isaac (Gen. 25:20-23), Joseph (Gen. 41:45), Moses (Exodus. 2:21), Boaz (Ruth. 4), Job (Job 1), Isaiah (Isa. 8:3), and Hosea (Hosea 3:1-3).
That should give you a good start on what God wanted and sanctioned. The ones who had multiple wives were living contrary to God's wishes and commands and it caused them problems.