In a scene from the movie “Lincoln,” a Democrat arguing against passage of the 13th Amendment derisively mentions the idea of interracial marriage to ridicule the legislation that would abolish slavery. A century-and-a-half later, as an African-American president is inaugurated for a second term, interracial unions still are relatively rare. And faith groups may be part of the reason Americans still find it so difficult to transcend race and ethnicity in matters of the heart, new research indicates.