On the contrary, mixing races leads to greater genetic fitness. By keeping a gene pool restricted to itself, you end up with diseases and birth defects. By mixing races, you have the potential to take the best traits from both and create something even greater.
Don't you agree, anon? Only by mixing races can we create the ubermensch. Take the athletic skill of a black man, the fair complexion and chiseled features of an anglo-saxon/nordic man, the intellectual skill of an Indian, the motor skill and lack of bodily odor from a Korean, the mercantile sense of the Jew, and the mathematical sense of the Arab, and you have made the perfect human being.
Besides, even your image agrees. The picture is named "pure man vs mixed subhuman", and indeed the inferior "pure" man is on the left while the superior mixed man is on the right.