Polygamy is biblical.
There isn’t a single commandment sanctioning it. There are ones about not multiplying wives to oneself, though.
Something is not automatically “biblical” by virtue of being mentioned in the Bible. By that logic, people might think that what Jephthah did to his daughter was acceptable to God when it was not.
Oh?
Then why did the Lord GOD command against it?
Deuteronomy 17:14-17
14 When thou art come unto the land which the LORD thy God giveth thee, and shalt possess it, and shalt dwell therein, and shalt say, I will set a king over me, like as all the nations that are about me;
15 Thou shalt in any wise set him king over thee, whom the LORD thy God shall choose: one from among thy brethren shalt thou set king over thee: thou mayest not set a stranger over thee, which is not thy brother.
16 But he shall not multiply horses to himself, nor cause the people to return to Egypt, to the end that he should multiply horses: forasmuch as the LORD hath said unto you, Ye shall henceforth return no more that way.
17 Neither shall he multiply wives to himself, that his heart turn not away: neither shall he greatly multiply to himself silver and gold.
It is MENTIONED in the bible; always negatively.
I could say that MURDER is 'biblical' as well.