Date: August 09, 2018 02:41PM
While on a mission in Boston in 1842-44, as a member of the 12 Apostles, Young had an affair with Augusta Adams Cobb, and she became pregnant, and left Boston, for Nauvoo, Illinois, where she married Young on November 2, 1843, and named the child she was pregnant with, George Brigham Cobb. The child died in 1843.
The reason this is adultery, and not just "spiritual wivery", is that Augusta was married to a living man, Henry Cobb, since 1822, at the time of the 1843 marriage to Brigham Young. They (Augusta / Henry) were not estranged or separated, etc., at the time Augusta had the affair with Young (a common excuse given by Mormon Apologists, in a attempt to avoid the adultery claim). Furthermore, Henry successfully sued to the Massachusetts State Supreme Court, in 1847, for divorce, on the grounds of adultery.
It is a matter of law and public record, that Brigham Young was an adulterer, as a Mormon Missionary and Apostle.https://www.findagrave.com/memorial/60955658
From the Supreme Court - Bostonhttp://www.truthandgrace.com/1847youngadultery.htm