D&C 132

 As the Lord addresses the doctrine of plural marriage, He explains - 

"God commanded Abraham, and Sarah gave Hagar to Abraham to wife. And why did she do it? Because this was the law; and from Hagar sprang many people. This, therefore, was fulfilling, among other things, the promises.

35 Was Abraham, therefore, under condemnation? Verily I say unto you, Nay; for I, the Lord, commanded it.

36 Abraham was commanded to offer his son Isaac; nevertheless, it was written: Thou shalt not kill. Abraham, however, did not refuse, and it was accounted unto him for righteousness."  (Doctrine and Covenants 132:34–36)

As He continues through this section, it becomes clear that the issue is not necessarily plural marriage, but obedience because "I, the Lord, commanded it"!

It is all about obedience to the word of the Lord at His timing.