For thousands of year, polygamy was practiced in Judaism. The Old Testament is chock full of it. The Germanic tribes practiced concubinage, though I have to admit, they only had one "wife" . The Romans definitely had concubinage, though again, only one wife. Having one wife in those cultures never meant exclusive sex with only that woman, as it came to be in later Christian cultures.
It is possible that my limited reading of history is incorrect, but AFAIK, the above statement is true. Please show me Roman Law forbidding sex with a woman not your wife, if you want an easy way to convince me that I am wrong about this.
Yet by 400 BC, religious Judaism characterized the practice of polygamy as a privilege of the patriarchs and restricted it to levirate marriages.
By the time of Christ, Israel had long been a monogamous society.
The Germanic tribes practiced concubinage, though I have to admit, they only had one "wife".
In ancient German and Roman society, as in modern US society, the wealthy and powerful often had "kept women" or concubines. Yet the moral standard of society was monogamy - if concubinage were considered legitimate and socially acceptable, then it would have been a legally recognized and lauded aspect of the culture.
The Romans definitely had concubinage, though again, only one wife. Having one wife in those cultures never meant exclusive sex with only that woman, as it came to be in later Christian cultures.
The lex Julia de adulteriis of 18 BC provides for severe penalties for both female and male adulterers - there is no provision in the law for legal concubinage. Moreover, that law was instituted to harmonize and recodify preexisting laws on the same subject.
This happened decades before the name of Christ was ever circulated in Rome.
These cultures all had prostitution, extramarital affairs, mistresses, nobles having a quickie with the scullery maid, etc. So did Christian Europe.
None of them had official recognition, endorsement or encouragement of plural marriage.