[George Q. Cannon]
The question also came up whether a white man who was married to a woman having negro blood in her veins could receive the Priesthood. I explained what President Taylor had taught me when I was a boy in Nauvoo concerning this matter; he had received it from the Prophet Joseph, who said that a man bearing the Priesthood who should marry or associate with a negress, or one of that seed, if the penalty of the law were executed upon him, he and her and the offspring would be killed; that it was contrary to the law of God for men bearing the Priesthood to have association with that seed. In this case submitted to us a white man had married a woman with negro blood in her ignorantly; yet if he were to receive the Priesthood and still continue his association with his wife the offspring of the marriage might make a claim or claims that would interfere with the purposes of the Lord and His curse upon the seed of Cain.
[George Q. Cannon, Journal, December 16, 1897, Church Historian's Press; Mormonr (mormonr.org/sources)]