If he marries another woman, he must not deprive the first one of her food, clothing and marital rights. If he does not provide her with these three things, she is to go free, without any payment of money.
If a man marries a woman who becomes displeasing to him because he finds something indecent about her, and he writes her a certificate of divorce, gives it to her and sends her from his house, and if after she leaves his house she becomes the wife of another man, and her second husband dislikes her and writes her a certificate of divorce, gives it to her and sends her from his house, or if he dies, then her first husband, who divorced her, is not allowed to marry her again after she has been defiled. That would be detestable in the eyes of the LORD. Do not bring sin upon the land the LORD your God is giving you as an inheritance.
With these scriptures in mind, the Pharisees came to Jesus to get his take on what constitued what “displeasing” meant. Some thought it only applied to sexual immorality others thought that wrinkles and burnt food counted. The Pharisees liked the latter definition over the former. To them Jesus said:
Some Pharisees came to him to test him. They asked, “Is it lawful for a man to divorce his wife for any and every reason?”
“Haven’t you read,” he replied, “that at the beginning the Creator ‘made them male and female,’ and said, ‘For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh’? So they are no longer two, but one. Therefore what God has joined together, let man not separate.”
“Why then,” they asked, “did Moses command that a man give his wife a certificate of divorce and send her away?”
Jesus replied, “Moses permitted you to divorce your wives because your hearts were hard. But it was not this way from the beginning. I tell you that anyone who divorces his wife, except for marital unfaithfulness, and marries another woman commits adultery.”
The disciples said to him, “If this is the situation between a husband and wife, it is better not to marry.”
Jesus replied, “Not everyone can accept this word, but only those to whom it has been given. For some are eunuchs because they were born that way; others were made that way by men; and others have renounced marriage because of the kingdom of heaven. The one who can accept this should accept it.”
There are a number of things set up in the Mosaic law that we would not want to live with and acting on them is not what we would call righteous. The rest of Exodus 21 talks all about selling off your daughter and how to treat slaves.
As you may noticed the Pharisees tried to say that divorce was commanded by Moses and Jesus corrects them. It was permitted because their hearts were hard. As polygamy and slavery are mentioned right there alongside divorce in Exodus 21, I don’t think we have to take a huge jump to conclude that they were also permitted because the Israelites hearts were hard (Further backed up by Paul calling elders to be the husband of but one wife). The rules governing them were provisions for remaining lawfully in the nation of Israel. They were not commandments for righteous living.
Modern day polygamist often claim that living out polygamy is an act of righteousness. I think Jesus quite clearly tells us that it is an act of hard-heartedness.
When God commanded Joseph Smith to resume polygamy was it because he was calling him to righteousness or hard-heartedness? Will we have hard hearts in the afterlife?