i dont think its fair to call this a cult. eri quickly realized they were in big trouble if they dont have a man around to protect them, so she does what any girl would do, give sexual favors in exchange for protection. she even said that if hes too horny theres not alot anyone can do, and to take out his frustration on her. so not only was she protecting the other girls (she doesnt know him well and he could be violent or rape them) she was establishing them to be able to survive in this crazy situation. now did the girls eventually fall in love with him? yes, obviously, because being with such a manly man would get any girls crotch twitching. especially when they found out how horrible they are treating the other students who got zapped there with them.
on that note, the release schedule for this manga is insane. like 5 chapters a year. its so hard to wait for new chapters because i really like this one. all the girls are 10/10 waifu material, and the mystery of where they are is really fascinating.