Read the book.
Polygamy cheapens all marriage, destroys families and turns half of our population into slaves.
It's a wicked, sinful, perverted institution.
Think of a list of attributes necessary to be, if not a good spouse, then at least not a bad one. I don't mean "plays piano" or "takes walks in the morning", I mean the baseline. "Not given to violent behavior". "Law abiding". "Not addicted to any behavior altering substances (such as alcoholics or meth users)". "Chaste (no sexual relationships outside of marriage)".
Thin the population with those standards, and you'll find more and more women, relative to the men. It won't be an awe-inspiring gulf, but there will be a disparity. I know a lot more women than men who had to lower their standards because they weren't married, wanted to be, and couldn't find anyone else interested. As I said, not a vast chasm, but a noticible pit certainly. Not every man of virtue would need to take a second or third wife, but a few would. Be ye not unequally yoked.
What many opponents of plural marriage forget is that, even at its height, polygamy wasn't practiced by that many people in Utah (iirc, 10%, maybe 20)