Brigham Young became aware of a black priesthood-holding Mormon man married to white LDS woman. Speaking of this interracial couple, he shared his thoughts about the couple and their child:
"If they [the couple and child] were far away from the Gentiles they wo[ul]d all have to be killed[.] [W]hen they mingle seed it is death to all."
"If a black man & white woman come to you & demand baptism can you deny them? [T]he law is their seed shall not be amalg[a]mated. Mulattoes are like mules[,] they cant have the children, but if they will be Eunuchs for the Kingdom of God's Heaven's sake they may have a place in the Temple."
Young states his belief that the couple's mixed-race child, or mulatto is like a mule, unable to reproduce. He points out that when such couples reproduce, "it is death to all" indicating either that all interracial families should be killed, or perhaps that the seed of mulattos that were able to reproduce would spread infertility through the population and eventually bring "death to all" (also believed at the time). He says the preferred solution would be to kill the family, but he fears gentile (non-Mormon) reprisal. He affirms interracial couples can be baptized, but could only have access to the temple as eunuchs (probably meaning celibate).
[Source: Clair Barrus, "The Policy on Gay Couples, and the Priesthood Ban: A Comparison", http://www.withoutend.org/policy-gay-couples-priesthood-ban-comparison/; LDS (or related) Documents on Walker Lewis, the Lowell, Mass. Branch of the Mormon Church and its missionaries and members, and the Priesthood Ban against Blacks, Compiled by Connell O'Donovan, http://people.ucsc.edu/~odonovan/Mormon_Chronology.html]