In the Old Testament polygamy was common, with no indication whatsoever that it was outlawed by the Decalogue. And divorce, which if not legal is naught but a special case of polygamy, was relatively easy:
“When a man takes a wife and marries her, if then she finds no favor in his eyes because he has found some indecency in her, and he writes her a certificate of divorce and puts it in her hand and sends her out of his house, and she departs out of his house, (Deu. 24:1)and of course Jesus famously addresses Mosaic divorce:
He said to them,
I would summarize the situation this way:
A case for a prohibition against polygamy is very strong, but not based on the Mosaic Law, which tolerated both polygamy and easy divorce. How could the ultimate, eternal moral law of God accommodate such a cavernous loophole?
It couldn't. But a shadow of the more fully revealed moral law as given by Jesus would necessarily be incomplete.