Why is it that the Lord never forbade polygamy?
Actually, he did in Genesis 1:27 and 2:24. In Matthew 19:2-9, Jesus cited those texts of Scripture to prove to the Pharisees that divorce-for-any-reason is Biblically not permissible. The way he framed that argument shows that these texts from Genesis also forbid polygamy.
Consider Mathew 19:2-9:
Pharisees came to him, testing him, and saying, "Is it lawful for a man to divorce his wife for any reason?" He answered, "Haven't you read that he who made them from the beginning 'made them male and female' [Genesis 1:27], and said, 'For this cause a man shall leave his father and mother, and shall join to his wife; and the two shall become one flesh' [Genesis 2:24]? So that they are no more two, but one flesh. What therefore God has joined together, don't let man tear apart." They asked him, "Why then did Moses command us to give her a bill of divorce, and divorce her?" He said to them, "Moses, because of the hardness of your hearts, allowed you to divorce your wives, but from the beginning it has not been so. I tell you that whoever divorces his wife, except for sexual immorality, and marries another, commits adultery; and he who marries her when she is divorced commits adultery."
Jesus establishes from Genesis 1:27 and 2:24 that marriage makes two persons into one flesh. Unless married persons have divorced lawfully (because one spouse is guilty of sexual infidelity as in Matthew 5:32 or one spouse is an unbeliever guilty of willful desertion that cannot be remedied as in 1 Cor. 7:13-15), which frees the innocent spouse to remarry, subsequent marriage of either to a third party is adultery against the first spouse. By the same reasoning, marriage to more than one wife (or husband) would also be adultery.
In 1 Corinthians 6:12-7:40, the Apostle Paul deals with questions surrounding "fornication" (sexual infidelity), marriage, and divorce on the basis of the same understanding of the one-flesh nature of marriage which Jesus sets forth in Matthew 5:32 and Matthew 19:2-89. On the basis of such Scripture, the Westminster Confession of Faith has this to say in Chapter 24, "Of Marriage and Divorce:
1. Marriage is to be between one man and one woman: neither is it lawful for any man to have more than one wife, nor for any woman to have more than one husband, at the same time....
3. It is lawful for all sorts of people to marry, who are able with judgment to give their consent. Yet it is the duty of Christians to marry only in the Lord....
6. Although the corruption of man be such as is apt to study arguments unduly to put asunder those whom God hath joined together in marriage: yet, nothing but adultery, or such willful desertion as can no way be remedied by the church, or civil magistrate, is cause sufficient of dissolving the bond of marriage....
I realize this may have raised new questions for you regarding marriage and divorce, and my reasoning may not be as clear as I hope. Do feel free to follow up with me. In the meantime, I pray the Spirit of Christ will lead you into a full understanding of all things taught in the Word of God.
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