Earth is not, nor has it ever been, the only planet where human beings live.
>sterile celibate
Absolutely not. They live a pure traditional, normal, rational life without extremism. Sex is much better and fulfilling there, AFAIK there are no divorces. This is easier to do on planets without demonic influences.
We had to forget to be able to fit in. There's no sense in external preaching, therefore Ground Crew was handpicked and incarnated on-site.