How does the LDS Church defend the racist doctrines of its past?

From 1849 to 1978, the LDS Church taught that [black people could not become priests](https://en.wikipedia.org/wiki/Black_people_and_Mormon_priesthood) . In 1863, [Young](https://en.wikipedia.org/wiki/Brigham_Young) stated: "Shall I tell you the law of God in regard to the African race? If the white man who belongs to the chosen seed mixes his blood with the seed of Cain, the penalty, under the law of God, is death on the spot. This will always be so." Brigham young (at least in my eyes) definitely considered this to be God's doctrine and teaching. Considering that this religion is meant to be God's revelation to the world, how does the LDS defend this racist teaching?