Polygamy is one of those things that's ok in small doses. But when it is practiced on a large scale it can really fuck things up. Lost boy syndrome, making the gene pool a lot more shallow, power struggles internally and among families, etc
Also, why is everyone assuming that polygamy = one man having multiple female slaves? Almost all of this thread is basing its reasoning off of that wrong assumption. It's completely voluntary, and it's perfectly possible for one woman to have multiple husbands. Nothing about supporting polygamy implies gender discrimination.
If anything, that "polygamy = one male dominating many females" world is more discriminatory to the men who now have to fight each other for the right to marry/breed. Women would at least get the freedom to choose whoever they want (or nobody), the joy of marriage, and the pleasure of being courted by many men, but most men would be stuck on their own....but of course we all know that women are the only victims in any society!