I have been wondering for a while about why polygamy and concubines were allowed. Why was God okay with this stuff?
We are not specifically told, but I can make some reasonable guesses. But to put it into context, as you were growing up were there not things that you did that your parents didn't like, wished you wouldn't do, but let slide because there were bigger issues they needed to address with you? It is sometimes called "picking your fights."
We do know that God has done this at times with the Israelites. He told the Israelites that He hates divorce (Malachi 2:16), but Jesus said God allowed it to happen in certain cases. "The Pharisees also came to Him, testing Him, and saying to Him, "Is it lawful for a man to divorce his wife for just any reason?" And He answered and said to them, "Have you not read that He who made them at the beginning 'made them male and female,' "and said, 'For this reason a man shall leave his father and mother and be joined to his wife, and the two shall become one flesh'? So then, they are no longer two but one flesh. Therefore what God has joined together, let not man separate." They said to Him, "Why then did Moses command to give a certificate of divorce, and to put her away?" He said to them, "Moses, because of the hardness of your hearts, permitted you to divorce your wives, but from the beginning it was not so. And I say to you, whoever divorces his wife, except for sexual immorality, and marries another, commits adultery; and whoever marries her who is divorced commits adultery."" (Matthew 19:3-9).
Notice the subtle wording. The Pharisees asked why Moses commanded divorce, but Jesus corrected them and said that divorce was not a command but an allowance. The allowance was given because of the stubbornness of the Israelites. In other words, God knew in advance that no matter what He said on this topic, the Israelites were going to divorce their spouses. Therefore, God chose to regulate it in order to minimize the damage it would cause. But, no matter how you look at it, God has never wanted it to be this way and He is not happy that it happens.
I suspect that polygamy is a similar case. Even though God said that marriage was between one man and one woman (Genesis 2:24), it is not long before we read of a wicked man taking two wives for himself (Genesis 4:19). It appears that the practice of polygamy became heavily entrenched in human society. We can see how stubborn people are about this in our own society. Polygamy has been outlawed in the United States for centuries, yet we still have people living in covert polygamous marriages despite the laws.
God chose to regulate it by demanding that each wife must be treated equally. "If he takes another wife, he shall not diminish her food, her clothing, and her marriage rights. And if he does not do these three for her, then she shall go out free, without paying money" (Exodus 21:10-11). If you consider the law, you would realize that in most cases it would be impossible for a man to do as God demanded, especially if he was already in a healthy marriage. As a result, the first wife had a ready reason to leave the marriage if she so chose, leaving the man with only one wife. A polygamous marriage could only exist if the first wife agree to it.
A concubine, by the way, is a slave who is taken as a wife. Having a concubine did not necessarily imply that the man had multiple wives, though it did happen in many cases.
Interestingly, God specifically forbade polygamy for certain individuals. Kings were not allowed to have multiple wives. "Neither shall he multiply wives for himself, lest his heart turn away; nor shall he greatly multiply silver and gold for himself" (Deuteronomy 17:17). Nor was a man allowed to marry his wife's sister (Leviticus 18:18). Yet, in both cases we know that the Israelites broke these laws over and over again.
Paul told the Athenians at one point, "Truly, these times of ignorance God overlooked, but now commands all men everywhere to repent, because He has appointed a day on which He will judge the world in righteousness by the Man whom He has ordained. He has given assurance of this to all by raising Him from the dead" (Acts 17:30-31). God made some allowances in the past because of the ignorance of mankind as a whole, but these times of overlooking faults is over. Everyone has access to God's word. Unlike the Old Testament, Christ's law applies to everyone. Jesus made right and wrong clear. "If I had not come and spoken to them, they would have no sin, but now they have no excuse for their sin" (John 15:22).
It is clear in the Bible that marriage is for one man and one woman. We know this is right and we will not seek a way around it.