Hendrik Hertzberg nails it in the most recent New Yorker:
You might think that an organization that for most of the first of its not yet two centuries of existence was the world’s most notorious proponent of startlingly unconventional forms of wedded bliss would be a little reticent about issuing orders to the rest of humanity specifying exactly who should be legally entitled to marry whom. But no. The Mormon Church—as anyone can attest who has ever answered the doorbell to find a pair of polite, persistent, adolescent “elders” standing on the stoop, tracts in hand—does not count reticence among the cardinal virtues. Nor does its own history of matrimonial excess bring a blush to its cheek.
It goes on and is definitely worth a read.