If I was slaughtering the hunter gatherers I would only kill the males, the females would make good wives for my new settled civilisation. And giving the men wives would enable the civilisation to maintain stability (incels will stab up the longhouse)
Males back then didn't fail to reproduce from the same social situations as today. They were not neets hiding in basements. Failure to reproduce basically means killed, exiled, or having your groups women taken (by force)