Polygamy is not promoted or condoned by the OT. There was nothing in the law about taking more than one wife or concubines. If fact, every case of polygamy in the OT led to negative consequences. Genesis 1 and 2 sets God’s standard for marriage. The rest of the Bible must be viewed in context of that. Polygamy in Genesis is always portrayed negatively, from Lamech (a representative of the evil of Cain’s descendents) onward.

Also, you need to understand the genre of literature the Bible was written in. The examples of polygamy are in the historical books. The historical books are presented as history. They’re recorded not because they’re promoting any characters as heroes we should emulate, but because that’s what happened. The only hero of the OT is God. Everyone else is presented as a messed up person, whom God used anyway. It’s history, not Aesop’s Fables. They’re not books of morality that provide us examples to follow. They’re the story of God’s covenant with Israel.

If the OT really promoted polygamy, why then did it disappear after the exile? Remember that after the exile, Israel stopped commonly practicing the sins for which God exiled them in the first place. Among those sins, apparently was polygamy. If the OT really promoted polygamy, why is it that Orthodox Jews today don’t practice it? If fact, they look down on it. To go further, it is actually in part, Israel’s influence on Western culture that causes us to look down on polygamy today.