Re: polygamy, Jesus specifically condemned divorce and remarriage as adultery, and taught that marriage was between one man and one woman, becoming one flesh. And that this had always been Godās plan, which He knew because He was God and was there. (Ditto on knowing why Moses had permitted divorce.)
It was a serious question, in early Christian circles, as to whether it was intrinsically immoral for a widow or widower to remarry at all, and only young kids were considered a good reason. Third marriages, after having a spouse die twice, were seen as ridiculously sex-hungry.
So if you are looking for Christianity to teach polygamyā¦ Noopers.