JFS says plural marriage is not essential to be member in good standing.
Joseph F. Smith, Testimony at Reed Smoot Hearing, March 2, 1904, in Michael Paulos, The Mormon Church on Trial: Transcript of the Reed Smoot Hearings (Salt Lake City: Signature Books, 2008), 67
I know that there are hundreds, of my own knowledge, who say they never did believe in it [plural marriage] and never did receive it, and they are members of the Church in good fellowship. Only the other day I heard a man, prominent among us, a man of wealth, too, say that he had received all the principles of Mormonism except plural marriage, and that he never had received it and could not see it. I myself heard him say it within the last ten days.