One of the biggest proofs that God is against polygamy is how Ishmael was not acceptable as Abraham’s son.
And if by “biblical” people mean that the Bible advocates something, then the slippery slope that they are on also might mean to them that the Bible advocates prostitution merely because Judah and Samson sought out prostitutes.
(s) so THAT is why there are so many wedding chappels in Las Vegas. (/sarcasm)