Jews distinguish between neighbors and others; the latter category includes animals. Ethical norms equally apply to both. The prohibition of murder, for example, equally applies to humans and animals. In that sense, Gentiles are similar to animals: it is not that Gentiles are low in Judaism, but that animals are high.

The Bible does not ban polygamy, but condemns it. Prophet Samuel condemns the behavior of kings, which includes having many wives, also condemned in Deuteronomy 17:17. Leviticus 18:18, which bans “taking your wife’s sister” probably means that all Jewish women are sisters, just like the Jews are sometimes called “your brother.” The Tenth Commandment speaks of a wife in the singular*. Similarly, Genesis 2:24: “a man shall leave his father and his mother, and be joined to his wife; and they shall become one flesh” establishes monogamy.
The Torah allows polygamy only in narrow cases. Exodus 21:10 speaks of Jewish concubines. Deuteronomy 21:15-17 speaks of children of the second “hated wife,” implying she is divorced. Levirate marriage (Deuteronomy 25:5-10) is a similarly specific case, and it does not even refer to polygamy but to one-time sexual intercourse.
The Torah regulates minute aspects of family relations, but is silent on the treatment of several wives, presuming such cases to be non-existent. Unlike other nations, Jews in the Land of Israel were monogamous.

The Torah admits and regulates evil rather than unrealistically demanding that it be abandoned. Seemingly barbaric teachings turn surprisingly progressive. Americans and Russians treated slaves worse in the nineteenth century than the Torah prescribed three thousand years ago. The Tenth Commandment prohibits jealousy in regards to various objects, equating wives with oxen, houses, and donkeys in that regard. That seemingly barbaric piece of sexism is actually extremely progressive. An ox was a matter of pride to ancient men, just like men’s cars are today. The house retained its role as the major symbol of male status. Hardly any man would speak disparagingly about his car or house, but many speak thus about their wives. The Torah’s listing of the wife among the most prized assets elevated the status of females from familial burden (as many men imagine their wives to be now) to an achievement a man should be proud of. Medieval poets praised women to the heavens, and frequented syphilis-ridden brothels. The Torah avoids unsustainable romanticizing but introduces a realistic guideline: treat your wife as well as your house.**

* The Tenth Commandment lists other items (house, donkey) also in the singular. They are mentioned, however, in a different clause from wife: “You will not covet your neighbor’s wife; and his slave, and his maid, and his ox, and his donkey, and anything which is your neighbor’s.” Regarding those other objects, the commandment states that not even one (thus singular) item could be coveted. Wife, unlike other objects, is mentioned in Hebrew in constructus, “your-neighbor’s-wife,” as a unique object with no implication that she is just one of a kind like a donkey.

** True, there is an important difference between wife and house: unlike a wife, a house cannot make your life miserable or divorce you. Though the Torah doesn’t directly allow women the right of divorce, they can always behave in such a manner as to make their husbands divorce them. Arguably, however, many women treat their husbands badly just because the husbands are unreasonably soft with them. Quite possibly, treating a wife as a highly prized object unquestionably owned by the husband improves familial relations greatly. In modern terms, that’s sexism, but I’m old-fashioned. Moreover, that’s exactly how we treat our small children. My guess is that most wives would gladly accept such assuredly high status instead of petty wrangling for dominance with their husbands.

judaism is never antiquated