Jeremy Runnells claims that prophets have contradicted one another on polygamy over time.
Jeremy Runnels, CES Letter: MY Search for Answers to my Mormon Doubts, self-published on his website: cesletter.org
3. POLYGAMY
Brigham Young taught the doctrine that polygamy is required for exaltation:
"The only men who become Gods, even the Sons of God, are those who enter into polygamy."
– Journal of Discourses 11:269 14
Several other prophets after Young, including Taylor, Woodruff, Snow, and Joseph F. Smith gave similar teachings that the New and Everlasting Covenant of plural marriage was doctrinal and essential for exaltation.
It’s even in the scriptures:
DOCTRINE & COVENANTS 132:415
“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.”
In a September 1998 Larry King Live interview 16, President Hinckley was asked about polygamy:
Larry King: “You condemn it [polygamy]?”
Hinckley: “I condemn it. Yes, as a practice, because I think it is not doctrinal.”
Contrary to President Hinckley’s statement, we still have Doctrine & Covenants 132 in our canonized scriptures. We're also still practicing plural marriage in the Temples by permitting men to be sealed to more than one woman (so long as only one is living). Apostles Elder Oaks, Elder Perry, and Elder Nelson are modern examples of LDS polygamists in that they're sealed to multiple women.
Polygamy is doctrinal. Polygamy is not doctrinal. Yesterday's doctrine is today's false doctrine. Yesterday's prophets are today's heretics.