I don't see the basis for this. an elementary fact in sexual reproduction is that greater variety in parental genes results in offspring with greater fitness. conversely, genes of greater similarity result in weaker offspring. this can be clearly seen in the results of inbreeding - offspring resulting from incest have a much higher rate of abnormalities and deficiencies. that is why almost all human cultures have an ingrained taboo against incest. this block against sexual attraction also extends to people who grew up together: https://en.wikipedia.org/wiki/Westermarck_effect this is because weak recessive genes are only expressed if they link up, which is much more likely in a situation of inbreeding. with heterozygosity, dominant alleles eliminate genetic disease, and new combinations can confer advantages. genetic diversity is beneficial, and different races will tend to have greater differences in genetics. that's why I would definitely encourage race-mixing in my eugenics ideology.