David O. McKay says polygamy "is not a principle but a practice" and that the blessings of "the eternity of the marriage covenant" are available to "a man with one wife."
David O. Mckay, First Presidency meeting, February 3, 1956, reprinted in Confidence Amid Change: The Presidential Diaries of David O. McKay, 1951-1970, ed. Harvard S. Heath (Salt Lake City: Signature Books, 2019), 136
I explained that it was my understanding regarding plural marriage that the having of more than one wife is not a principle but a practice. The principle of the eternity of the marriage covenant revealed to the Prophet [Joseph Smith] and all the blessings pertaining to that may be obtained by a man with one wife.