I dont really think that they had sex, I mean they are so flustered about the date and so nerveous that they will kiss at the end. Would be tottaly nonsense if they had sex before that. Guess Ai is just cluesless and simply dont see their romance, so she asks herself what has happen in the bed. I guess both girls just slept, close at best. It was just.. real and right for them, so their love can bloom.