Would not practicing Polygamy fall under the category of copying the customs of gentiles? A certain mainstream religion makes a point of having one and only one wife. Are Jews technically copying that gentile custom, by practicing monogamy?
A quick glance through the Tanach would show that for the most part, monogamy was the norm. While polygamy was not considered immoral, it was still not regularly practiced.
In fact, whenever you come across one of the biblical heroes marrying a second wife, it is always with a special reason and with the wife permission. This is besides for the case of Jacob, which was a trick to begin with.
To see the attitude of the Torah you can see how God spoke to Noah in Genesis 7:2, where he refers to animal couples as 'a man and his wife'.
This continues into the Talmud as well. There is no Talmudic sage, who comes to mind, who had two wives at once. On the contrary, the word for a co-wife is צרה, which is explained as literally being a pain. And so, it is obvious that it was never considered a very positive type of relationship.
I like where you are going with this.
But, generally the prohibition against copying the customs of the gentiles does not apply to something that has a rational basis. See Rama Yoreh Deah 178:1: וכל זה אינו אסור אלא בדבר שנהגו בו העובדי כוכבים לשם פריצות . . . או בדבר שנהגו למנהג ולחוק ואין טעם בדבר דאיכא למיחש ביה משום דרכי האמורי ושיש בו שמץ עבודת כוכבים מאבותיהם אבל דבר שנהגו לתועלת . . . מותר
This is of course assuming there's a rational basis for monogamy :)