Here's the best explanation I've seen:
The question then arises, �how is it that the Old T. personages were able to have more than 1 wife?�. God allowed them to have more than 1 wife but He did not bless it. They chose to be disobedient to God and take more than 1 wife. Look at the problems that each of the men who took more than 1 wife had. From Abraham to Solomon, all of them had major problems because of their extra wives. It was not God�s will yet they chose it and paid the price for it. So, just because the men chose to disobey, does not mean that God approved of their actions.
there are several examples in the OT where God commands widows to marry brother of their husband, what if brother is already married? Seems like there are other examples where God told someone to marry an additional woman but can't recall exactly who it was and not 100% sure I'm correct.
The command was to brother-in-law's to volunteer to take care of their brother's wife and to carry on the property rights for him and to keep his family line in tact in Israel. It is called a Levirate marraige.
I think Deut 25 is the only passage that deals with it directly, but we can see in practice where this went on in Judah's house in Genesis 38 and with Ruth in Ruth 3. In the new testament Matthew 22 speaks on this as well in a round about way.
Deuteronomy 25:5-10
5 If brothers are living together and one of them dies without a son, his widow must not marry outside the family. Her husband�s brother shall take her and marry her and fulfill the duty of a brother-in-law to her. 6 The first son she bears shall carry on the name of the dead brother so that his name will not be blotted out from Israel.
7 However, if a man does not want to marry his brother�s wife, she shall go to the elders at the town gate and say, �My husband�s brother refuses to carry on his brother�s name in Israel. He will not fulfill the duty of a brother-in-law to me.� 8 Then the elders of his town shall summon him and talk to him. If he persists in saying, �I do not want to marry her,� 9 his brother�s widow shall go up to him in the presence of the elders, take off one of his sandals, spit in his face and say, �This is what is done to the man who will not build up his brother�s family line.� 10 That man�s line shall be known in Israel as The Family of the Unsandaled.
