The state's interest in marriage is stability. Generally speaking polygamy does not work for stability. Inherent in the whole polygamous movement is a deep and abiding misogyny and denigration of women. So polygamy is objectionable on lots of grounds.
If there is one realm in which it is essential to be sublime it is in wickedness. You spit on a petty thief but you can't deny a kind of respect for the great criminal.