I'm still wrapping my head around what happened during that whole sequence. In the end, it was the actual Dragons that enabled the whole NTR shit with Barigesu. It also says something about human's greed/sexual appetite/aggression that empowers the dragons, though I might be wrong about this; even with transliteration on chiitrans its still kinda hard to piece stuff together.
In the end, killing the Mother Dragon turns off humanity's aggression, AND their sexual appetite. Meaning everyone turns barren AND uninterested in sex. Humanity was doomed to die off eventually, but there's a strange sort of peace, with everyone cooled off.
Your choice at the very end, when Ina talks to you, determines their fate. Its either someone manages to give birth despite everything (or it might have been Beherunihito's kid with Imosu, I'm not sure myself), and she gives back the Mother Dragon's soul to be reborn, thus reigniting humanity and starting the whole Ryuunin vs Demon King shit again, OR humans eventually die off and Beherunihito's the only one left in the world.
The end sequence has Beherunihito hoping that someone in the future manages to break the whole cycle of NTR and grief, because she's tired of dying and repeating the whole thing all over again.