Christ was plurally married:
(I discover that some of the Eastern papers represent me as a great blasphemer, because I said, in my lecture on marriage, at our last conference that Jesus Christ was married at Cana of Galilee, that Mary, Martha, and others were his wives, and that he begat children. All that I have to say in reply to that charge is this—they worship a Savior that is too pure and holy to fulfill the commands of his Father. I worship one that is just pure and holy enough “to fulfill all righteousness;” not only the righteous law of baptism, but the still more righteous and important law “to multiply and replenish the earth.” Startle not at this, for even the Father himself honored that law by coming down to Mary, without a natural body, and begetting a son; and if Jesus begat children he only “did that which he had seen his Father do.” (Apostle Orson Hyde, J.D. 2:210)
And so was Abraham the Father of our Covenant but is this Celestial Law found within the Book of Mormon? The Book of Mormon is a book “for our day”, and we are as a Church under condemnation (D&C 84) for rejecting the Covenant found with it. Doubtless this is for more than one reason, and it becomes paramount to our hopes of exaltation if not salvation to find out all these reasons so we may repent.
So when the Law of Celestial Marriage is found strewn about the Book of Mormon such as in 2 Nephi 8:1 “Hearken unto me, ye that follow after righteousness. Look unto the rock from whence ye are hewn, and to the hole of the pit from whence ye are digged.
Look unto Abraham? How can we look unto Abraham?
Certainly there is much in the Bible and Pearl of Great Price to glean from, but also the Doctrine and Covenants which we have Covenanted to Obey:
132: 30 Abraham received promises concerning his seed, and of the fruit of his loins-from whose loins ye are, namely, my servant Joseph-which were to continue so long as they were in the world; and as touching Abraham and his seed, out of the world they should continue; both in the world and out of the world should they continue as innumerable as the stars; or, if ye were to count the sand upon the seashore ye could not number them.
31 This promise is yours also, because ye are of Abraham, and the promise was made unto Abraham; and by this law is the continuation of the works of my Father, wherein he glorifieth himself.
32 Go ye, therefore, and do the works of Abraham; enter ye into my law and ye shall be saved.
33 But if ye enter not into my law ye cannot receive the promise of my Father, which he made unto Abraham.
34 God commanded Abraham, and Sarah gave Hagar to Abraham to wife. And why did she do it? Because this was the law; and from Hagar sprang many people. This, therefore, was fulfilling, among other things, the promises.
35 Was Abraham, therefore, under condemnation? Verily I say unto you, Nay; for I, the Lord, commanded it.
So there we have it. The Book of Mormon asks us to look unto Abraham. But should we is the question? For obvious reasons it’s a good thing in the Book of Mormon Jacob condemns the unauthorized practice of plural marriage (see Jacob 2), and those inside the Covenant are keeping this law in part as we can see as defined by Joseph F. Smith here:
Some people have supposed that the doctrine of plural marriage was a sort of superfluity, or non-essential to the salvation of 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. . . . 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 of marriage in part . . . . 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 . . . . it is useless to tell me that there is no blessing attached to obedience to the law (polygamy) or that a man with only one wife can obtain as great reward, glory or kingdom as he can with more than one, being equally faithful. Patriarchal marriage involves conditions, responsibilities and obligations. . . . Man . . . cannot receive the fullness of the blessings unless he fulfills the law, any more than he can claim the gift of the Holy Ghost after he is baptized without the laying on of hands by the proper authority, or the remission of sins without baptism. 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. (Joseph F. Smith, JD 20:23-31)
So when, according to prophecy are we to keep this law of Abraham in full and again bless all women who otherwise haven’t access to a righteous Priesthood holder? Many women today live singly not being able to find a suitable Priesthood bearer and many more are counseled to stay with an unrighteous man against their better judgement, still others are not able to use their agency to bear children based on the disapproval of the head of household. When and why did it cease is maybe a more pertinent question? After all it certainly wasn’t to ‘obey the law of the land’. At LDS.org we read: “In Joseph Smith’s time, monogamy was the only legal form of marriage in the United States. Joseph knew the practice of plural marriage would stir up public ire.”, and it wasn’t because of the 1890 Manifesto, ” new plural marriages were performed between 1890 and 1904, especially in Mexico and Canada…”, “The statements emphasized that the Church practiced no marital law other than monogamy while implicitly leaving open the possibility that individuals, under direction of God’s living prophet, might do so.”23 (https://www.lds.org/topics/plural-marriage-in-kirtland-and-nauvoo?lang=eng#23
Perhaps then, our agency is the hinge as described by our Lord Jesus Christ, “Have I not given my word in great plainness on this subject? Yet have not great numbers of my people been negligent in the observance of my law and the keeping of my commandments, and yet have I borne with them these many years; and this because of their weakness—because of the perilous times, and furthermore, it is more pleasing to me that men should use their free agency in regard to these matters. Nevertheless, I the Lord do not change and my word and my covenants and my law do not, and as I have heretofore said by my servant Joseph: 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, for it is everlasting, and those who will enter into my glory must obey the conditions thereof; even so, Amen.” Revelation Received Monday, September 27, 1886, at Centerville, Utah Territory. (John Taylor Papers, LDS Archives).