The parallel society idea is cool, but the problem with polygamy is it leaves a subset of men without wives. The FLDS amd other Polygamous Mormon offshoots have a consistent problem with young men leaving eventually, since there aren't enough women for everyone to be polygamous.

In ancient times where polygamy existed, this was rectified by going to war and taking brides as spoils, but even that will run dry, as there's only so many nations to plunder before you control the surrounding area.