From a Catholic perspective, heresy.
More broadly, of all nations that could be called "Christian" now or in the past--Protestant, Catholic, Orthodox--I know of none save for the United States, post-Revolutionary France, and Haiti, that imposed any racial qualifications for marriage as a matter of civil law (there are reasons that so much of the population south of the U.S. border is mestizo), nor wrote racial segregation into their civil codes, until the late 19th/early 20th century. And in the U.S., both of these institutions, ironically, trace back to the literally un-churched and un-schooled settlement that grew out of the Virginia tidewater, where Africans were once treated on par with English in indentured servitude... until the rulers of the colony discovered slavery was more profitable.
But then again, what do you expect out of folks whose response to "you shall love your neighbor as yourself" is "sez who?"