@komichi:You would usually break off but no one wants to break off with the other party feeling sad/depressed and there's also the courage issue...
and since they are in the middle/high school(should be high but not sure) days, their issues are more or less what we face in everyday life, cause if someone likes a person of the opposite gender, if the other party requests to have sex/intimate actions, chances of them resisting is close to 0% so we can't really label Sekai as loose or anything since it was her first time with Itou.
But yes, I agree that her ways of getting Itou is wrong but nevertheless, was she even thinking clearly at that time?