Well, listen...I'm no fan of the LDS church, but you're obviously not very well educated in terms of the Mormons.
1. The LDS church banned polygamy centuries ago.
2. Polygamy is practiced by many religions.
3. The motivations for LDS polygamy were clearly the need to produce children quickly, and also to make up for the fact that there were not, at the time, enough adult male members of the church.
4. While splinter, fringe sects who have broken away from the LDS church still practice polygamy, and while the question you ask about their motivations are both on point, attacking the Mormon faith for the behaviors of these splinter groups is silly and intellectually dishonest.