The following quote against race-mixing is from Monsignor Glenn's book Sociology, which was given a nihil obstat by the Censor Departus of the Church and received an imprimatur from the Bishop of Columbus. Monsignor Glenn was Professor of Philosophy and later President of the College-Seminary of St. Charles Borromeo. The text would presumably have been used in seminaries for the instruction of priests-to-be.

"That there are lines between classes of people is a certainty that may as well be acknowledged at once, and the color-line is one of them. While it is entirely possible to ignore these lines, the social effect of such action is seldom happy. Just as a member of the true Church is earnestly dissuaded from marriage, by dispensation, with a non-member, so should a member of one race be dissuaded from marriage with a person of another color. In marriages of either type there is a definite injustice done to children, there are almost inevitable misunderstandings between the parties themselves, and there is sure to be some friction between the families so gracelessly united."
-- Sociology, p. 372