D&C 132 - The Law of Sarah
The author of The CES Letter apparently does not understand the law of Sarah.
In D&C 132:65, the law of Sarah is discussed only as it applies to the wife of the keyholder (the "one" man holding the sealing keys).
If she (Emma) accepts plural marriage, she guides future polygamous additions to the family. If she rejects plural marriage, the husband is not bound to remain monogamous due to her disobedience. Emma rejected plural marriage in Kirtland, Ohio, in 1836. If Emma had rejected plural marriage when presented in Nauvoo, she would have been under condemnation (D&C 132:65). Consequently, delaying that introduction was advantageous to both her and Joseph.
Emma accepted plural marriage, giving Joseph four wives and later approving a fifth, but she also vacillated in her support.
The last nine months of the Prophet’s life, Emma regulated his plural marriage family in the privacy of the Nauvoo Mansion where several plural wives lived together with them.
Joseph Smith’s Polygamy: Toward a Better Understanding by Laura Hales