President Joseph F. Smith (Journal of Discourses 20:28-29)
Some people have supposed that the doctrine of plural marriage was a sort of superfluity, or non-essential, to the salvation or exaltation 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 eternity, will receive an exaltation as great and glorious, if he is faithful, as he possibly could with more than one. I want here to enter my solemn protest against this idea, for I know it is false. There is no blessing promised except upon conditions, and no blessing can be obtained by mankind except by faithful compliance with the conditions, or law, upon which the same is promised. The marriage of one woman to a man for time and eternity by the sealing power, according to the will of God, is a fulfillment of the celestial law of marriage in part--and is good so far as it goes--and so far as a man abides these conditions of the law, he will receive his reward therefore, and this reward, or blessing, he could not obtain on any other grounds or conditions. But this is only the beginning of the law, not the whole of it. Therefore, whoever has imagined that he could obtain the fullness of the blessings pertaining to this celestial law, by complying with only a portion of its conditions, has deceived himself. He cannot do it.
President Brigham Young (Wilford Woodruff Journal, Typescript 7:152)
...[men] who did not have but one wife in the Resurrection that woman will not be his but [will be] taken from him and given to another.
President Wilford Woodruff (Utah Stake Historical Record #64904/CH0/1877-1888. Quarterly Conference held March 3rd and 4th, 1883; Sunday, 2 PM, p.271)
The new and everlasting Covenant is marriage, plural marriage - men may say that with their single marriage the same promises and blessings had been granted, why cannot I attain to as much as with three or four, many question me in this manner I suppose they are afraid of Edmunds, what is the Covenant?
It is the eternity of the marriage covenant, and includes a plurality of wives and takes both to make the law...Joseph Smith declared that all who became heirs of God and joint heirs of Christ must obey his law or they cannot enter into the fullness and if they do not they may loose the one talent, when men are offered knowledge and they refuse it they will be damned and there is not a man that is sealed by this priesthood by covenants to enter into the fullness of the law and the same with the woman she says she will observe all that pertains to the new and everlasting Covenant both are under the Covenant - and must obey if they wish to enter into a continuation of the lives or of the seeds.
President Brigham Young (Journal of Discourses 9:322) (Deseret News, 14 November 1855)
If any of you will deny the plurality of wives and continue to do so, I promise that you will be damned.
President Brigham Young:
Monogamy, or restrictions by law to one wife, is no part of the economy of Heaven among men.
President Joseph F. Smith (Journal of Discourses 21:10)
This doctrine of plural marriage is one of the most important doctrines ever revealed to man. Without it man would come to a full stop; without it we never could be exalted to associate with and become Gods, neither could we attain to the power of eternal increase.
President John Taylor (Matthew Cowley, Life of Wilford Woodruff, p.542 -- as quoted in MHP 1:311-312)
If we do not embrace that principle soon (plural marriage), the keys will be turned against us. If we do not keep the same law that our heavenly father has kept, we cannot go with him. A man obeying a lower law is not qualified to preside over those who keep a higher law.
Whatever the truth-claims of the Mormon Church may be (which is not the subject of discussion here), it is plain that the contemporary Church is not teaching the same doctrines as its founder and early leaders. To deny these early teachings is to cast a dark shadow of doubt over the inspired nature of the teachings and revelations of Joseph Smith, Brigham Young, Joseph F. Smith, and others - to acknowledge them is to be at odds with the current leadership and risk certain excommunication. Either way, the modern Mormon must deny his conscience and lie to the Spirit of God when forced to suppress the simple, plain facts. Indeed, he must either remain in the modern LDS Church and abandon all hope of entering the Celestial Kingdom (since he is denied the means to do so, viz. polygamy), become a fundamentalist Mormon (who still practice celestial marriage), or abandon Mormonism altogether, for no other choices face him.