“All unrighteousness is sin,” 1 John 5:17. Every violation of the divine law is sin, so we can answer this question by looking at God’s laws on marriage.
The divine law blessed marriage between one man and one woman from the beginning and intended it to be permanent. God gave Adam one helper, not helpers. “. . . a man shall. . .be joined to his wife; and they shall become one flesh.” This divine arrangement has never changed. Genesis 2:20-24, 27-28 NASB
And when God started civilization over, He chose four couples. “Noah, and Shem, and Ham, and Japheth, the sons of Noah, and Noah’s wife, and the three wives of his sons with them . . .” Genesis 6:8-10; 7:7, 9, 13, 15
Nevertheless, in the Old Testament, polygamy was permitted for a time. (See Genesis 30:26.) For example Abraham, Jacob, David, Solomon, all had multiple wives. Please note that these polygamous unions resulted in marital grief 100% of the time! God may have permitted man to engage in these unsanctioned behaviors in order to learn that God’s arrangements are always the best. Recall that Hagar was a concubine given to Abraham by his wife, Sarah. God provided for Hagar. An angel told her; “. . . You will give birth to a son, Ishmael . . . for the LORD has heard of your misery.” But it caused great unhappiness for Abraham, Sarah, Hagar and the children, and God sent Hagar away. God comforted Abraham in his sadness. Then God said to Abraham; “And as for Ishmael, I have heard you: I will surely bless him . . . I will make him into a great nation.” Genesis 16:11 & 17:18-20 NIV
God told Israel; the King “must not take many wives, or his heart will be led astray,” Deuteronomy 17:14, 17 NIV. Verses 18 through 20 go on to say this was "the Law.” A study of the lives of men like King David and King Solomon will show that many of their problems were due to their polygamous relationships. Sorrow always follows sin and is the consequence for not obeying God’s Law.
Jesus often began his teachings by saying; “Ye have heard that it was said to them of old time…but I say to you…” In this way, Jesus supported the Old Testament and God’s original divine arrangement. Jesus’ teachings on marriage are in Matthew 5:27-32, Matthew 19:3-12, Mark 10:1-12 and Luke 16:15-18. His teachings do not include polygamy as a choice. But the habit of polygamy was still practiced at that time. Consequently, Paul spoke plainly to the churches. Paul told them that their pastors must be the husband of only one wife. 1 Timothy 3:2, 12