my kneejerk theory is selfishness - familial continuity does not constitute personal gain in their eyes (also shortsightedness because this also isn't true, atheists really out here thinkin they niggas not gonna get old one day)
not wanting to bring kids into a screwed up world is a dishonest cope, I don't think anyone sincerely believes that
and often it involves misanthropic pseudo-environmentalism, meaning more kids = kill le mother earth, so no white babbies for you goy
