Why wasn't Solomon's polygamy sinful? I know that Solomon was a king, but why doesn't the Bible describe his polygamy as sinful? Was he allowed to commit sins others could not because he was a king? I thought God showed no partiality.
Although the narrator in Kings and Chronicles may not say so, Deuteronomy does say it was a sin, in Deuteronomy 17:15-17


  [15] you shall surely set a king over you whom the Lord your God
  chooses; one from among your brethren you shall set as king over you;
  you may not set a foreigner over you, who is not your brother. [16] But
  he shall not multiply horses for himself, nor cause the people to
  return to Egypt to multiply horses, for the Lord has said to you, ‘You
  shall not return that way again.’ [17] Neither shall he multiply wives
  for himself, lest his heart turn away; nor shall he greatly multiply
  silver and gold for himself. (NKJV)


Solomon broke all these provisions, multiplying horses, wives, and gold and silver.


  All King Solomon’s drinking vessels were gold, and all the vessels of the House of the Forest of Lebanon were pure gold. Not one was silver, for this was accounted as nothing in the days of Solomon.  (1 Kings 10:21 NKJV)


Accounted as nothing because he had so much of it.