Jedediah Grant explained to Thomas Kane that polygamy was necessary because there were more women than men.
Jedidiah M. Grant to Brigham Young and First Presidency, December 30, 1851, 4-5, CR 1234 1, Church History Library
In the Propagation of our principals and the gathering together of those who imbraced them, it was found after nine years experience, that the aggregate number of Females was three to two males, showing that one third of our women must remain single, or marry out of the Church.
Either was thought impolite. The Prophet Joseph Smith was led therefore to inquire of the great great Abraham who is and ever has been the disposer of this matter. Counting from time to time special Dispenations according to the situations and circumstances of the People to whom said Dispensations there given; The Dispensation given by the Lord through the Prophet, to the Church of Jesus Christ of Latter Day Saints & one limited and strict in its nature; Like unto the one given unto Abraham & others in his day And not as Elastic is the Dispensation appears to have been in the days of David & Soltomon.