I do not know what Bible you are referring to, but it is not the one used by Christians. That monogamy was the condition that was intended for mankind is evidenced by Genesis 2:24, "For this reason a man will leave his father and mother and be united to his wife, and they will become one flesh." The context is that of Adam and Eve in the Garden of Eden, right after Eve was created from the body of Adam by supernatural means. The first case of polygamy recorded in the Scriptures had to do with a descendant of the infamous Cain: Lamech, who "took unto him two wives" (Gen. 4:19).
Some argue that the Old Testament condones polygamy by citing the examples of Kings David and Solomon. Yet David's adultery with Bathsheba led to his committing murder of Bathsheba's husband, the death of a son by his relationship with her, and God placing a curse on his household. Deuteronomy 17:17 states that a king is not to take many wives, as it would lead his heart astray. While King Solomon was certainly a polygamist, I Kings 11:3 indicates that his multiple wives did lead him astray, including the worship of false gods.
The New Testament specifically denounces polygamy. In Matthew 19:4-6, Jesus Christ repeats the Genesis 2 language on marriage being between a man and a woman as context for His statement that divorce is forbidden except in cases of adultery. The Apostle Paul, in Romans 7:1-4, terms a woman who is married to another man while her husband is still alive an adultress. In I Corinthians 7:1-2, he states that every man should have his own wife and every wife her own husband, except for those cases where a person chooses celibacy. In both Titus and I Timothy, Paul avers that deacons and elders are to be the husbands of only one wife.
Church teachings have historically forbidden polygamy. As far back as the second century AD, documents such as the Shepherd of Hermas (150 AD) and Justin Martyr condemn polygamy as well as divorce without cause. Church councils in the fourth century and afterward condemned it, as did the church fathers such as Augustine of Hippo. Protestants who revolted against Rome were equally condemning of polygamy. The Second Helvetic Confession of Faith condemns the practice. The Westminster Confession of Faith states: "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." The legal codes of historically Christian nations - Catholic, Orthodox, and Protestant - forbade polygamy and prohibited divorce, except in cases of adultery and desertion.
Both Scripture and the practice and teaching of the Christian churches from the very beginning have clearly and repeatedly condemned polygamy.
Polygamy was regulated, not forbidden.
Exodus 21:10: "If he takes to himself another woman", :Deuteronomy 21:15-17 "If a man have two wives".
The rights of the firstborn went to whatever son was born first, regardless of which woman bore this child (Deuteronomy 21:15-17).
Each wife was to be treated equally. Equal food, clothing, and affection. (Exodus 21:10).
If a man died without children then his brother was obliged to marry the widow. (Deuteronomy 25 vv 7-10). There is nothing to suggest that this was limited to unmarried brothers, is there?
I don't see a prohibition in the NT. Rather, a nudging toward monogamy.
Now, you can argue things changed. Socially, they did. The economic benifits of polygamy have been shattered. That, and only that, accounts for its present demise. Believe me, the New Testament would come under some reinterpretation on the matter should the need arise.