As far as two, four or six consenting adults living together, I myself don't care. But for Utah to become a State they had to outlaw it per Federal Law.
There were four acts of congress that were passed in Utah prior to the one that granted them statehood.
1862 the Anti-Bigamy Act
1874 Poland Act
1882 Edmunds Act
1887 Edmunds-Tucker Act
The Edmunds-Tucker Act was said by some set up to destroy the LDS if it continued to practice polygamy. Up to and including disbanning the Church and selling it's assests, prohibting woman from voting, making voters provide a signed document that they deny polygamy and a few others. As you can guess they agreed to the State hood provisions instead which granted them freedom from the Edmunds-Tucker act yet still deny polygamy.
It's not really the act of polygamy per say that is illegal in many Mormans eyes, it's telling people and living an open life. If you have a duplex and are 'married' to the neighbor, no one says anything. If you have a 'wife' and introduce her as your cousin but everyone knows you are living together, that's okay, it's telling people that gets them in trouble. That's not of course taking into consideration the sects that are doing what they want.
The reason the sects even exist and live the way they do is the Mormon doctrine states that in order to enter the highest heaven that those who enter must be living in polygamy. Mormon doctrine also states that they believe in being subject to the laws of the land, insofar as those laws protect individual rights. So for the time being, the largest Mormon sect, the LDS church, has decided it is more important to live the laws of the land than those of God. Other Mormon faiths have made a different choice in the matter, and practice polygamy in defiance of the laws of the land.
As for why the Federal government made polygamy illegal, it was public outcry that forced the U.S government to make a stand on polygamy. By other religious leaders. It was outlawed in 1862 and re tried in the case of Reynolds vs U.S in 1878 and upheld that it was not against the constitution to ban it. Reynolds vs U.S stated that religious duty was not a defense when you are under a criminal indictment. In short, you can't make religious doctrine that overrides the law of the land. Because though the congress can't legislate against opinion, it can against an action, of which polygamy is.
I am descended from polygamist. My great great grandfather had 3 wives. It is not very uncommon if you had family from Utah to find many people who are descended from polygamist. As far as I know, going by records I have, none of the woman were under the age of consent for the time, the youngest being 17. I'm not sure of the legal age of consent to marriage in the 1800's but it even states that right now it can be younger than 18 if not forced or the male is 10 years or older than the female. I do know that the age of sexual consent was just raised from 14 to 16 in 2005 in Utah.
