Henry L. Stilton, US Secretary of War, recalling his attempt to persuade Archibald MacLeish not to speak out against the Army's discrimination against blacks, wrote the following in his diary on January 24, 1942:
I pointed out that what these foolish leaders of the colored race are seeking is at the bottom social equality, and I pointed out the basic impossibility of social equality because of the impossibility of race mixture by marriage. - Henry L. Stimson, US Secretary of War
Reference: Personal Justice Denied: Report of the Commission on Wartime Relocation and Internment of Civilians, part 1, chapter 1, page 46.
Looking at the two "impossibilities" separately: In the context of the social environment in the United States, it is easy to understand the "impossibility of social equality", but what I don't understand is the "impossibility of race mixture by marriage".
From my understanding, historical (and contemporary) European-Americans believe in race as such: if person of "race A" marries person of "race B", and they produce offspring, the children will be considered "mixed race". In light of this, why did he believe in the "impossibility of race mixture by marriage"?
Also, why did he believe that the "impossibility of social equality" is due to "the impossibility of race mixture by marriage"?
(Note: I am rather new to the history of the Americas).