“From the days of the Prophet Joseph even until now, it has been the doctrine of the Church, never questioned by any of the Church leaders, that the Negroes are not entitled to the full blessings of the Gospel...

Furthermore, your ideas, as we understand them, appear to contemplate the intermarriage of the Negro and White races, a concept which has heretofore been most repugnant to most normal-minded people from the ancient patriarchs till now. God’s rule for Israel, His Chosen People, has been endogamous” 

“We are not unmindful of the fact that there is a growing tendency, particularly among some educators, as it manifests itself in this area, toward the breaking down of race barriers in the matter of intermarriage between whites and blacks, but it does not have the sanction of the Church and is contrary to Church doctrine.”

Letter from First Presidency to Dr Lowry Nelson

(George Albert Smith, J. Reuben Clark, David O. McKay)

https://archive.org/stream/LowryNelson1stPresidencyExchange/Lowry_Nelson_1st_Presidency_Exchange#page/n5/mode/1up

Note: This quote is only included to illustrate assumptions around the ban.