In defense of polygamy....
It's illegal (now) in most WESTERN countries. By Shar'ia (Islamic religious law) it is permitted (although monogamous relations are also acceptable by religious doctrine, and more socially acceptable these days).
The marrying of very young women (often socially/psychologically coerced) is technically wrong under most religious codes, although that rarely stops men from doing it. (I will point out that from a historical perspective, women in Western countries often married young; you were an 'old maid' if you weren't married or engaged by 19-20!)
It's safe to say that only the most dramatic cases get media coverage. A polygamous relationship (legal or not) where the women are equal in the marriage to the man and everyone is happy is unlikely to get reported to the authorities. It would be more correct to say that polygamy is one type of polyamorous relationship.
A term for the concept 'polyamory' (where the multiple men/women are equal partners in the relationship) seems to be fairly new. (As opposed to the idea itself, which I suspect may be rather old.)
Polyandry, although rare even historically, did occur (Mongolia, I think).
In short, every single possible type of relationship possible between men and women (and men and men, and women and women...) has been tried at some point in human history. Different societies have regarded different types of unions as 'natural' and others as 'forbidden'.
What polyamory is doing now is confronting a societal belief that this type of relationship is 'wrong'. Opposing commonly-held beliefs always results in conflict and controversy.