Wondering if anyone has any materials on "multiple marriages" in the Old Testament? Would like to hear your views on why this was tolerated among God's people. I understand this was not God's choice, however there is no outcry from the prophets against such. The practice of multiple wives and concubines in the Old Testament is part of a class I am teaching on The World of The Bible.
Why polygamy is in the Old Testament is a matter of conjecture. In Deuteronomy 25:5-10 God commands that a man marry his brother's childless widow without regard to his current marital status. Given that it was a command of God would suggest to me that God expected it to be followed. That sounds far beyond just toleration to me. Also in I Kings 15:5, God sums up David's life by saying he did everything correct except for the matter of Uriah. If you leave out Bathsheba, David still had his own host of wives and concubines. That would easily explain why the prophets don't speak out against it. Polygamy was allowed and in some cases expected.
You also elude to "this was not God's choice". I suspect that you arrive at that conclusion from things that Jesus said in Matthew 19. Jesus' comment in Matthew 19:8 actually says that God permitted divorce because their hearts were hard. He does not say that God permitted multiple marriages because their hearts were hard. God gives his opinion on divorce here (and in Malachi 2:16), but there is no comment on what he thinks about polygamy. There is only an implication given that marriage was intended to be between one man and one woman in Matthew 19:4-5 by the statements that mankind was made male and female and that the two (one man and one woman) become one flesh.
To find what is required in the New Testament we need passages like Ephesians 5:33, I Timothy 3:2, and I Timothy 3:12. Here we find that the direction of the church was to be one husband for one wife. Why the shift? I don't know, but it is obvious that there is a shift.
I don't know that there is anything useful to be gained by speculating on why God would allow polygamy in the Old Testament and not allow it in the New. We don't always get an answer on "Why?" I could ask why God forbade that one fruit to Adam and Eve, but he does not forbid us any fruit. I expect that it has to do with seeing if we will follow His word more than because there is inherent harm in the action. Certainly most things that are identified as sin have a negative physical impact, but that does not mean all things are that way. Some things are wrong only because God said they are wrong. God is free to have one covenant where an action is required for religious purposes (like circumcision - Leviticus 12:3) and the other where it is condemned if done for religious purposes (Galatians 5:1-4). I know of several people who try to find a health benefit to circumcision, but it was not given for health reasons, it was given as part of that older covenant.
So it is with polygamy. There may be logical social benefits that one could derive that would explain "why" to a level of satisfaction of our intellect. For example, "they did not have another good system for taking care of widows in an age where men died young". I don't personally like that reasoning because that implies that God was incapable of devising another rule whereby the widows could be taken care of. He chose that way for that time and we should respect it for what it was.