The girls seem insane to us but remember, in that world there would only be 200 million men, with 7.8 billion women. Keeping that in mind, every girls behavior makes sense
why are you trying to make predictions based on a different manga? It doesn't matter if that is the same author's work. It was far lower in quality than Usotsuki Satsuki and the author clearly learnt his/her lessons from it.