True Mormonism! Not fundamentalism.
(this one's for you, Actionlad!)
What people fail to realize is how modern Mormonism barely escaped having this movie as its reality!
First, a little history.
Joseph Smith, the first founder of Mormonism, had 34 wives. 11 of those wives were married to other men. That wasn't a problem for Joe - he received a "revelation" and sent them to faraway lands as missionaries. Then he could swoop in and have his way with them. He also had 7 wives who were, by today's standards, underage. The youngest was 14, who was underage by 1830s standards. His first polygamous wife (i.e., wife #2), was Fanny Alger, a 16-year-old girl who he was enjoying when she was his live-in housekeeper. His first wife Emma almost committed suicide when she caught them in the act. Old Joe also liked sisters, he married 3 pairs. He even had a mother-daughter set of wives. He was sexually advanced for his time. Eventually he received a "revelation" that polygamy was to be called "the new and everlasting covenant of marriage". It would be practiced openly by the Mormon church until 1890.
Now, before any true-believing Mormons say "Now, now Mr. Bender! He didn't have any sex with those women!", you should know this - polygamy was only to be practiced to "raise up seed" to a righteous leader. If you just had wives to say "hey dude, check out all my hot wives!" that was an abomination.
We see this from the Book of Mormon, specifically, Jacob, Chapter 2:
23 But the word of God burdens me because of your grosser crimes. For behold, thus saith the Lord: This people begin to wax in iniquity; they understand not the scriptures, for they seek to excuse themselves in committing whoredoms, because of the things which were written concerning David, and Solomon his son.
24 Behold, David and Solomon truly had many wives and concubines, which thing was abominable before me, saith the Lord.
25 Wherefore, thus saith the Lord, I have led this people forth out of the land of Jerusalem, by the power of mine arm, that I might raise up unto me a righteous branch from the fruit of the loins of Joseph.
26 Wherefore, I the Lord God will not suffer that this people shall do like unto them of old.
27 Wherefore, my brethren, hear me, and hearken to the word of the Lord: For there shall not any man among you have save it be one wife; and concubines he shall have none;
28 For I, the Lord God, delight in the chastity of women. And whoredoms are an abomination before me; thus saith the Lord of Hosts.
29 Wherefore, this people shall keep my commandments, saith the Lord of Hosts, or cursed be the land for their sakes.
30 For if I will, saith the Lord of Hosts, raise up seed unto me, I will command my people; otherwise they shall hearken unto these things.
Yeah, I'm sorry everyone. That's super boring to read. But it's what the Mormon god put down as scripture when Joe Smith wrote the Book of Mormon. You can see that he says David and Solomon's many wives were abominable, but "this people" - or the people in the Book of Mormon, should not do this. So they were commanded to have 1 wife, and no concubines... UNLESS - God wants a bunch of kids (i.e., raise up seed unto me).
In other words, if Joseph was told by god to take multiple wives, it was to "raise up seed". If Joseph was not planting that seed, he was committing an abomination.
So I guess it was a good thing that Joseph Smith could get a revelation stating that polygamy was A-OK. Though he did get this "revelation" after he'd been caught a few times. You'd think as a "prophet" that he would have seen this coming and had that revelation ready when he got caught with the 16-year-old by his first wife. But I digress.
Now after Joseph was killed, other Mormon leaders practiced polygamy. Brigham Young had 55 documented marriages - the youngest was 16 (he was 55). Wilford Woodruff had only 10 - the youngest was 26 (he was 78). Lorenzo Snow had 9 wives - his youngest was 15 (he was a spry 57). And Joseph F. Smith only had 2. But after a while his first wife left him. So I guess he ended up with one. Oh well, it's the thought that counts.
Polygamy was officially disavowed in 1890 (with a "revelation"), but really not cracking down until after 1904. This was due to extreme pressure from the US Government, even to the point of eliminating the church as an entity and sending troops. However, this is not to say it stopped altogether. Other groups continued to practice, and a mainstream Mormon "apostle" was even excommunicated for this in the 1940s. Even today, a man can be "sealed" to more than one woman. If you're married to one woman and she dies, you can be "sealed" to another. This way you get both of these ladies for eternity.
Other Mormon leaders have said over the years that this is coming back at some point, like when Jesus reappears, or when everyone is super righteous. The church has done its best to whitewash these statements.
Anyhoo, these so-called "fundamentalist" Mormons are just doing what Mormons do. They all believe in the Book of Mormon, and have Joseph Smith as a prophet. You can bet that the rest of the Mormon church would still be doing this today if it was allowed. Obedience is everything in the Mormon church.
http://wivesofjosephsmith.org/
http://www.lds.org/topics/plural-marriage-and-families-in-early-utah
https://en.wikipedia.org/wiki/Mormonism_and_polygamy
http://www.mormoninfographics.com/2012/09/the-many-wives-of-joseph-smi th.html
http://www.mormoninfographics.com/2012/09/the-wives-of-brigham-young.h tml
http://www.mormoninfographics.com/2013/08/dirty-old-men-early-mormon-p rophets.html