The mainstream Church of Jesus Christ of Latter-Day Saints has disavowed polygamy for over a century.
To imply that “some sects believe in polygamy” is not unlike implying that all Baptists practice snake-handling and arsenic-drinking. Yes, extremely small enclaves of individuals who claim similar descent practice such heterodox acts, but they have been completely estranged from the mainstream church for generations.
I suppose you aren’t familiar with the LDS Scripture called the Doctrine and Covenants section 132? I wonder why the LDS refuse to live the everlasting covenant?
http://scriptures.lds.org/en/dc/132
3 Therefore, prepare thy heart to receive and obey the instructions which I am about to give unto you; for all those who have this law revealed unto them must obey the same.
4 For behold, I reveal unto you a new and an everlasting covenant; and if ye abide not that covenant, then are ye damned; for no one can reject this covenant and be permitted to enter into my glory.
61 And again, as pertaining to the law of the priesthood—if any man espouse a virgin, and desire to espouse another, and the first give her consent, and if he espouse the second, and they are virgins, and have vowed to no other man, then is he justified; he cannot commit adultery for they are given unto him; for he cannot commit adultery with that that belongeth unto him and to no one else.
62 And if he have ten virgins given unto him by this law, he cannot commit adultery, for they belong to him, and they are given unto him; therefore is he justified.
63 But if one or either of the ten virgins, after she is espoused, shall be with another man, she has committed adultery, and shall be destroyed; for they are given unto him to multiply and replenish the earth, according to my commandment, and to fulfil the promise which was given by my Father before the foundation of the world, and for their exaltation in the eternal worlds, that they may bear the souls of men; for herein is the work of my Father continued, that he may be glorified.
The Lds church has never disavowed eternal polygamy [Lds message that their polygamists live in Kolob Heights, where they are "raising up seed"; plus Lds apostle Bruce McConkie said in his "Mormon Doctrine" book that the "holy practice" of earthly polygamy would be re-instituted when the Mormon jesus returns]
Also, time-wise note: The Lds church was still secretly "plural marrying" hundreds of couples from 1890-1910 -- some of whom were still alive in the early 1960s. (The Lds church rarely tried to break up these "plural unions" that were formed by 1910).