Genuinely curious: how does he justify his sexual preference with eugenics? From the pov of eugenics he's a dead end thus not improving the quality of the human population.

You got it the other way around, the question is why shouldn't he be pro eugenics? There's absolutely zero benefit to being "heterosexual" when you can be sexually attracted to males and still get your dick hard to breed a woman, in fact that same "heterosexuality" aka disguised misandry and cuckoldry is what will lead to the fall of any civilized society.