From what I understand. Kyo is a typical dude tryna get laid, Eri is a parasitic douche nozzle and Fuyuki is a unlucky confused person that makes poor decisions. Before people start saying Eri isn’t that bad, think about it. Dates/latches onto anybody that gives her any kind of affection and...
They didn’t do the deed. The more likely reason she is in her underwear is because how hammered she was and vomited. Pretty common thing in the world of alcoholic beverages and getting drunk.