Demonstrating how women in polygamist marriages are oppressed with credible sources would help your argument.
I'm not willing to devote that effort to be honest. More that view is just off the many stories of polygamists religious organizations in which generally it seems that it is very young women with a very old husband.
I've also read how often runaway young girls/women end up turning to prostitution for economic survival - however hopefully there is a way out of that. If, instead, those girls turned to financially secure old men for his harem, they become trapped then by children of that polygamy.
The other example I would give is the highly restricted and gender-controlled roles that women in polygamy marriages such as Muslim seem to be. Don't most societies that have polygamy tend to deny women rights even to vote, extreme punishments for wrong-gender behavior and so forth?
Because wealth will buy wives, the trend would be old men with wealth and young women without wealth. The result would be a society of grandfather-less children and widowed mothers - in which men marry old and women marry young.
Polygamist cultures don't seem historically to be cultures also with great personal freedom. I would think the rates of divorce of the women from their rapidly aging husbands would do anything but create a stable family and would frequently lead to adultery and divorce by the women.
As a different point, the idea that few people would actually opt for polygamy seems just an assertion. If polygamy became popular and given it does tend to be one man and many wives, what of all the other men? There is a point at which society shouldn't have to, or just won't, respect the rights to power by wealthy while outlawing power by virtual of strength or numbers. There is only inherent validity to wealth equating a protected power.
I assume by polygamy, though, you also would have no problem of a woman having multiple husbands. Would your logic then also extend to group-marriages - many women and many men all in the same "marriage?"