I will submit that normalization of polygamy was caused by the acceptance of it. The gender imbalance (if any actually existed) due to warring was caused by raiding for women because they didn’t have enough women to go around.
Childbirth mortality would naturally create a shortage of women, compounding it with polygamy results in a surplus of young men best gotten rid of by sending them out to raid.
It was a self-perpetuating problem. Polygamy is socially dysfunctional and is a huge driver of conflict and violence.
Go out West to the areas with large unreformed Mormon populations.
The first thing that happens in polygamous societies is you have to get rid of a lot of the young men. They often get force out of the area at a quite young age in order to limit the competition for young women.