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.
2. ROMAN. Adulterium properly signifies, in the Roman law, the offence committed by a man, married or unmarried, having sexual intercourse with another man's wife. Stuprum (called by the Greeks fqorav) signifies the commerce with a widow or a virgin. It was the condition of the female which determined the legal character of adultery; there was no adultery unless the female was married. It is stated, however (Dig.48 tit.5 s13), that a woman might commit adultery whether she was "justa uxor sive injusta," the meaning of which is not quite certain; but probably it means whether she was living in a marriage recognised as a marriage by the Roman law or merely by the jus gentium. The male who committed adultery was adulter, the female was adultera. The Latin writers were puzzled about the etymology of the word adulterium; but if we look to its various significations besides that of illegal sexual commerce, we may safely refer it to the same root as that which appears in adultus. The notion is that of "growing to," "fixing," or "fastening to," one thing on another and extraneous thing: hence, among other meanings, the Romans used adulterium and adulteratio as we use the word "adulteration," to express the corrupting of a thing by mixing something with it of less value.