The Nazis were not as anti-sexual or for monogamy, as many think. In fact, quite a number of conservative catholics disliked them for their lenient sexual morality.
It was basically all about breeding: Create as much offspring, of a high genetic quality, the right racial features. They even had breeding facilities, Ordensburgen, where selected men and women would have sex to create children with the desired features. They called the act "decken", a word normally used for letting animals have sex, for breeding.
On the other hand, those considered unfit were subject to culling out, "ausmerzen", a word also originating from animal breeding. Retards were sterilized, later, many were used as test subjects for what to do with the Jews. Those who were ugly, didn't fit into society or would never be able to get a woman, had some chance to board a train and take a shower upon arrival. Except if they had a high enough rank.