I think their mindset is just human must stick together no matter it take. Thats why she didnt even realise he can switch side.Such passionate words from a naked man wearing a condom on his head...
Though in all seriousness, in what world did the human girls actually think he'd listen to them at this point? Sure, he's put up with their non-stop abuse so far thanks to his own idiocy and them realizing that he's their only real hope, but they practically abuse him every other page when there isn't a fight happening.
Why would he side with them? Especially now that their cruelty towards the demon girls is shown so openly?