Polygamy
Jesus was clear that the model for marriage is laid out in Genesis 2 (Mt.19:4). He expected the Pharisees to understand God’s law on divorce based on this chapter, even though divorce is never mentioned in Genesis 2. God’s design for marriage was that the two were to become one flesh. If it is God’s desire, Jesus reasoned, for the two to be one flesh, then it is not His desire for them to be torn apart and become two again. Taking the same approach to interpreting the words of Gn.2, the fact that marriage is the union of 2 into one would necessarily exclude a third party. 
The first instance of polygamy appears in Gn.4:23, which is a section that shows the damage done by the fall. The man characterized by pride, revenge, and murder is also the first polygamist. 
Next comes Abraham’s decision to bring Hagar into his marriage bed, which is certainly portrayed as a bad thing. Not only was it a perversion of the promise God had made, but it created strife within the home between Sarah and Hagar. 
Isaac was monogamous, but Jacob not only married two women, but he did so in a manner that would later be expressly forbidden in the law. While there is no direct statement in the Mosaic law forbidding polygamy per se, there is a prohibition against marrying sisters. 
Leviticus 18:18 Do not take your wife's sister as a rival wife and have sexual relations with her while your wife is living.
In the account of Jacob’s marriage to Leah and Rachel (Gn.29) there is no comment in the narrative condemning what he did – it is merely reported. 
Like Sarah and Hagar, Leah and Rachel also experienced conflict between each other as rivals, as did Peninnah and Hannah (1 Sam.1:1-7). Solomon’s wives led him astray. Indeed, every instance of polygamy in Scripture, where details of the relationship are given, are problematic. There are places where it is mentioned without comment, but where Polygamy is described in the home it is consistently seen in a negative light.
Polygamy is never endorsed as good, and when marriage is spoken of in the wisdom literature, law, and prophets there is always an assumption of one wife. The ideal marital relationship portrayed in the Song of Songs is between a man and a woman. In Pr.5:18 and Mal.2:14,15, men are called to remain faithful to the wife (not wives) of their youth. While there is one place in the law that regulates polygamy (Dt.21:15ff), most often the assumption is that there will be one wife.
Some questions, however, remain. Why is there no direct prohibition of polygamy in the law? Why is it not punished by God? Polygamy may have been a part of the ancient culture, but the Lord did not hesitate to forbid other things that were common in the surrounding cultures.
It seems that while an astute student of God’s Word could have figured out from Genesis that the ideal model for marriage was one man and one woman, there simply is no prohibition of polygamy in Scripture. God permitted it.
And if you struggle with that, just remember the concept of progressive revelation. God did not reveal everything at once. As far as we know in the Garden of Eden there was only one command. And bit by bit God revealed more and more of His law and His character. Obviously during the time of Cain and Able there was no law against marrying a close relative, but later that law was put in place. There are things that weren’t in the Law that show up later in the Prophets. And there are things that never were revealed at all in the Old Testament and appear for the first time in the New Testament. In fact, in 1 Cor.7 Paul reveals some things about marriage that Jesus didn’t even reveal. The doctrine of progressive revelation shows us that God did not reveal everything at once. And one of the things that was not explicitly stated in Old Testament times was the importance of monogamy. 
Why? One possibility is the fact that those were brutal days, and a lot of men died in battle. Single women had a very difficult time supporting themselves in that culture (to say the least!), and so perhaps it was an act of mercy when God allowed certain wealthy, powerful men to take care of more than one woman. 
Another possible reason it was not mentioned in the Law was the fact that it was not a widespread problem. Most men were not wealthy enough to have more than one wife, which is why the average Israelite man had only one wife. Kings could afford many wives, and there is a command in the law directed at them.
A third possibility is that God wanted to reserve a way to escalate the importance of the one flesh union of marriage in New Testament times because the picture of the relationship between Christ and the Church is so crucially important in the New Testament era. Because that picture is so vital, it is fitting that there be an increase in what God revealed about the ideal marriage relationship. Also, since in the New Testament the body becomes the Temple of the Spirit, sexual activity becomes more significant. (1 Cor.6:18-20)