The blueprint was set in Eden, when God gave Adam one wife, not two.
Kings were warned about this in Deuteronomy 17:17. Multiplying wives would drag their hearts away. But we watched it happen anyway.
The legal angle first. If it’s against the law where you live, then that alone makes it sinful for a believer (Romans 13 territory). Start there, before even getting into the biblical design questions.
The demographic side gets overlooked though. Wars killed off huge numbers of young men, and some places had two or three women per man.
Being a widow or unmarried woman in a patriarchal society was brutal. No safety net, no rights. Polygamy was often the better option given the alternative.
Solomon, though. Good example, since he gets called out for straying from God’s path (the many wives, the love of wealth). But the stuff before Christ was Judaism anyway.
The Christian standard comes from Jesus pointing back to one man and one woman as lifelong partners.