Question: Why did Jewish men have multiple wives in the past?
Answer: In its early Israelite practice, Judaism permitted a man to have more than one wife. A woman, however, could only have one husband. And adultery was defined as a married woman having sexual relations with a man other than her husband.
Throughout the world, it was common for kings to have multiple wives. This practice was politically wise because he helped the kingdom to establish relationships with surrounding countries. Jewish kings followed suit and also took multiple wives. However, this often led to religious problems as too often the foreign wives of the Jewish kings brought foreign idolatrous worship into the Jewish palace. In various places in the Hebrew Bible, this problem is discussed.
Then, in the 11th century, Rabbenu Gershom, in a major amendment to Jewish tradition, prohibited the practice of marrying more than one wife. It is believed that the amendment was an effort to prevent anti-Semitism. There was a fear that Christian men would convert to Judaism in order to have more than one wife, and then the Church would feel threatened and fight Judaism. The law achieved its purpose of preventing a rise in conversion and resulting anti-Semitism, and it also held over time.