OFFICIAL STATEMENT: “God expects us to uphold and keep His commandments regardless of divergent opinions or trends in society. His law of chastity is clear: sexual relations are proper only between a man and a woman who are legally and lawfully wedded as husband and wife.” -from the LDS Church’s official “Response to the Supreme Court Decision Legalizing Same-Sex Marriage in the United States,” by The Council of the First Presidency and Quorum of the Twelve, 29 June 2015. [source]
CHURCH ESSAY: “In Joseph Smith’s time, monogamy was the only legal form of marriage in the United States. … Latter-day Saints distinguished between sealings for time and eternity and sealings for eternity only. Sealings for time and eternity included commitments and relationships during this life, generally including the possibility of sexual relations. Eternity-only sealings indicated relationships in the next life alone. Evidence indicates that Joseph Smith participated in both types of sealings.” -from Plural Marriage in Kirtland and Nauvoo, an essay available in the Gospel Topics section of LDS.org. [source]
COMMENTARY: Either today’s prophets are lying about God’s law of chastity or else Joseph Smith was an adulterer. It seems pretty clear cut to me. But I don’t want to present a false dichotomy here, so another possible option is that God’s law of chastity changed sometime after Joseph Smith’s death and the prophet who received that revelation didn’t think it was important to tell any of us about it. After all, violations of the law of chastity are second only to murder in seriousness, so it’s not like it’s that important. But what do I know? I’m just another apostate.