The Old Testament mentions that many Patriarchs had numerous wives. The Lord says to David via Nathan: I gave your master's house to you, and your master's wives into your arms (2 Sam 12:8). How are we to understand polygamy and when did God forbid it?
When God first established marriage, it is clear that his vision was that: a man leaves his father and mother and clings to his wife, and they become one flesh (Genesis 2:24). Hence there is one man, with one wife, and the two are stably united by “clinging” to one another as the text says. Hence divorce and multiple wives were not part of God’s design of marriage.
However, on account of human sinfulness, and out of fear that men would kill their wives to be free to marry another, Moses allowed divorce. It is also clear that the customs of the Ancient Near East also infected Israel’s notion of marriage and that many, at least wealthier men and patriarchs, did often take more than one wife. Thus we see that sin corrupted what God intended and that, for a time, God overlooked this sinful behavior.
However, we ought not equate the mere reporting of sinful behavior with approval of it. For, while the polygamy of the Patriarchs is reported, so is all the trouble it caused wherein brothers of different mothers contended and even killed one another. For example, there are terrible stories told of the sons of Gideon, and also the sons of Jacob, to mention but two. The well-known story of Joseph being sold into slavery by his brothers emerges from the internecine conflicts of brothers of different mothers. Hence while reporting polygamy, the Bible also teaches of the evil it brings forth.
Gradually God led the ancient Jews from approving of polygamy such that, by the time of Jesus, it was rare. As for divorce, Jesus sets it aside by teaching the people at that time although Moses allowed divorce due to their hard hearts, it was not this way at the beginning and it was now time to return to God’s original plan (cf Matt 19:4, 8) and that we should not separate what God has joined.
As for Nathan saying that God gave David his many wives, this can be understood as the ancient tendency to stress God as the primary cause of all things. It does not necessary mean that God actively wanted and approved of polygamy, only that he is the first cause of everything that exists and happens.
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