Why the FLDS live Polygny

Joseph F Smith (founders nephew and future President of LDS church) JD 20:28 -
“When that principle was revealed to the Prophet Joseph Smith, he very naturally shrank, in his feelings, from the responsibilities thereby imposed upon him; foreseeing, as he did in part, the apparently insurmountable difficulties in the way of establishing it, in the face of popular opinion, the traditions and customs of many generations, the frowns, ridicule, slander, opposition and persecution of the world. Yes, this man of God, who dared to meet the opposition of the whole world with bold and fearless front, who dared to dispute the religious authority and accumulated learning and wisdom of the age - who dared everything for the truth, and shrank not even from the sacrifice of his own life in testimony of his divine mission, shrank, in his feelings, from the weight of the responsibility of inaugurating and establishing this new innovation upon the established customs of the world. But he did not falter, although it was not until an angel of God, with a drawn sword, stood before him and commanded that he should enter into the practice of that principle, or he should be utterly destroyed, or rejected, that he moved forward to reveal and establish that doctrine.”

Founder Joseph Smith (Contributor 5:259) -
“They accuse me of Polygamy, and being a false prophet, and many other things which I do not now remember; but I am no false prophet; I am no imposter; I have had no dark revelations; I have had no revelations of the devil; I made no revelations; I have got nothing up of myself. The same God that has thus far dictated me and directed me and strengthened me in this work, gave me this revelation and commandment on celestial and plural marriage, and the same God commanded me to obey it. He said to me that unless I accepted it, and introduced it, and practiced it, I, together with my people, would be damned and cut off from this time henceforth. And they say if I do so, they will kill me! Oh, what shall I do? If I do not practice it, I shall be damned with my people. If I do teach it, and practice it, and urge it, they say they will kill me, and I know they will. But said He, we have got to observe it. It is an eternal principle and was given by way of commandment and not by way of instruction.”

Heber C. Kimball, one of the original 12 Apostles and First Councilor to Brigham Young(10/12/1856 JD 5:203) -
“You might as well deny “Mormonism” and turn away from it, as to oppose the plurality of wives. Let the Presidency of this Church, and the twelve Apostles, and all the authorities unite and say with one voice that they will oppose that doctrine, and the whole of them would be damned.”

Joseph F Smith (JD 20:28 7/7/1878) -
"Some people have supposed that the doctrine of plural marriage was a sort of superfluity or non-essential to the salvation of mankind. In other words, some of the saints have said and believe that a man with one wife, sealed to him by the authority of the Priesthood for time and all eternity, will receive an exaltation as great and glorious, if he is faithful, as he possibly could with more than one. I wish here to enter my solemn protest against this idea for I know it is false…. The marriage of one woman to a man for time and eternity by the sealing power, according to the law of God is a fulfillment of the Celestial law in part… But this is only the beginning of the law, and not the whole of it. Therefore, whoever has imagined that he could obtain the fullness of the blessing pertaining to the Celestial Law, by complying with only a portion of its conditions has deceived himself He cannot do it…. I understand the law of Celestial marriage to mean that every man in this church who has the ability to obey and practice it in righteousness, and will not, shall be damned. I say I understand it to mean this and nothing less, and I testify in the name of Jesus that it does mean that.”

John Taylor Third Prophet and President of the Church (2/1/1885 JD 26:152)-
"I would like to obey and place myself in subjection to every law of man. What then? Am I to disobey the law of God? Has any man a right to control my conscience, or your conscience, or to tell me I shall believe this or believe the other, or reject this or reject the other? No man has a right to do it. These principles are sacred, and the forefathers of this nation felt so and so proclaimed it in the Constitution of the United States, and said "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof." Now, I believe they have violated that, and have violated their oaths, those that have engaged in these things and passed that law, and those that are seeking to carry it out. Congress and the President of the United States and the Judiciary, and all administrators of the law are as much bound by that instrument as I am and as you are, and have sworn to maintain it inviolate. It is for them to settle these matters between themselves and their God. That is my faith in relation to this matter. Yet by their action they are interfering with my rights, my liberty and my religion, and with those sacred principles that bind me to my God, to my family, to my wives and my children; and shall I be recreant to all these noble principles that ought to guide and govern men? No, Never! No, NEVER! NO, NEVER! I can endure more than I have done, and all that God will enable me to endure, I can die for the truth; but I cannot as an honorable man disobey my God at their behest, forsake my wives and my children, and trample these holy and eternal obligations under foot, that God has given me to keep, and which reach into the eternities that are to come. I won't do it, so help me, God."

John Taylor, from Wilford Woodruff's diary (Life of Wilford Woodruff pg.542) -
“If we do not embrace that principle soon, the keys will be turned against us. If we do not keep the same law that our Heavenly Father kept (Celestial and Plural marriage) WE CANNOT GO WITH HIM. A man obeying a lower law is not qualified to preside over those who keep a higher law.”

Shortly before his death, while he was in hiding from the Federal government John Taylor recorded a revelation from God. Dated September 26, 1886, In it says:
“All those who would enter into My glory must and shall obey My law. And have I not commanded men that if they were Abraham’s seed and would enter into my glory, they must do the works of Abraham. I HAVE NOT REVOKED THIS LAW NOR WILL I, it is everlasting, and those who will enter into my glory MUST obey the conditions thereof.”

Esther E. E. Morrison (my great grandmother) -
"At the time the manifesto was signed, I was staying with Mary [her older sister] I remember so vividly when the word came about this, as father, mother, Charlie’s father, and mother were in Mary’s home. Everything was so quiet, and it seemed as though there was a feeling of some terrible happening taking place. It was felt in the home and outside even in my young years, about 8 or 9 years old, and not understanding the great significance of what happened, I felt the dreadful impact of it.
Years later, about 1925, I attended Daughters of the Pioneers meeting held at the home of Mrs. Joseph J Danes, on Fifth east, who was a daughter of President Wilford Woodruff. Also another daughter, Mrs. Beatty was there. She spoke on the Manifesto. During her talk she said:

“The day father signed the manifesto I was in the front room when he came home. I shall never forget his face. He moaned, and said “O God what have I done?” He immediately went upstairs to his room. He remained there for a whole week, never speaking to any of us. When he came down and joined the family he looked awful. The ravages of sickness could never have made him look worse. It was awful.”

As a note here, my children, I desire to leave this to you. I know the law of plural marriage to be a heaven truth. Your father was an issue of that great law. You have a righteous heritage. Don’t trample on it, but treasure it, and accept the pattern given to you.”

If you want to read an interesting history of Mormon Polygny
READ THIS

The most interesting note in there is this:
1897 Sept, Wilford Woodruff, married 6th wife, Lydia Mountford. (President of the church, married Lydia a year before he died) This was 7 years AFTER he wrote the "Manifesto" ending polygamy.