I am not Jewish; I did major in religious studies in college before switching right before graduation to accounting (when I realized I need a job). In my studies, I took several courses in Judaic theology and wrote my senior paper on the beliefs of one Jewish sect. So, with that caveat, here is what many Jews believe(d).
The Bible has always said marriage "should be" between one man and one woman". However, it provided exceptions.
It starts in Genesis where God create "one" woman for Adam. Often to understand the Bible, especially when reading the Old Testament, one needs to understand Genesis.
So, what are the exceptions? One example is a man was obligated to take his brother's wife as an additional "wife", if his brother dies. If his brother did not have children, then the man was obligated to have a child with his brother's widow and that child would be seen as an extension of his brother's blood line.
The focus on continuing a family bloodline is pretty powerful in many religions. In Judaism if a man's wife was unable to conceive, he had an obligation to have a child with a "concubine".
Another reason for marrying the wife of a dead brother and adopting his children was to provide them a social safety net and limit society's obligation to those widows and orphans that had nowhere to turn.
Jews cite the "commandment" by God to be fruitful and multiply as not an option but a requirement of men. So, having more than wife or having a child with a concubine was considered "blessed by God" only if it was to fulfill an "obligation", not to pursue pleasure.
Many people reading the Bible today point to Soloman and his thousand wives and concubines. The don't understand the Jews are mocking that relationship in the Old Testament as an example of depravity. God showed his displeasure in the history that followed Soloman's reign.
As you move to the New Testament, Paul, who was a devout Jew who had studied the law extensively, highlights his understanding that ideally people should be single so they can focus on God and not have the distractions of a family, but if they choose to marry, they should follow the law of one man and one woman.
None of this suggests that people back then or today follow what Jews believe to be God's law. Humans have sinful natures and as such it is "natural" for humans to pursue "unholy" relationships.
However, because all humans sin, rather than focus on the sins of others, we should focus on our own. Many people religious or not, want to focus on others and whether that involves calling them sinners or "canceling" them for not being politically correct, it amounts to the same thing... declaring oneself as a judge rather than recognizing oneself as a sinner.
