William Clayton had learned of plural marriage at least by March 7, 1843, when Joseph Smith told Brigham Young to give Clayton a "favor" regarding priesthood instruction. The word "favor" in Clayton's journal refers to the granting of an additional wife. Clayton and his first wife, Ruth Moon, were in their seventh year of marriage and had three children. The prophet personally visited the family in their Nauvoo home and suggested that Clayton participate in plural marriage. Margaret Moon, his legal wife's sister, became Clayton's first plural wife. The marriage was recorded on April 27, 1843, three months before Smith dictated his plural marriage revelation.
[Source: Joseph Smith Polygamy Timeline, http://www.i4m.com/think/polygamy/JS_Polygamy_Timeline.htm]