well, given how she is towards other people my guess about her not being a virgin is: given how hot and sexy she looks, some dickhead thought she was easy prey and went after her. sure, since she doesnt understand anything about emotions or sex she agreed to the bastard, the guy getting the easy ok from her for some fun went and did the deed (once, twice, etc) but given how emotionless she is it wasnt as fun as the guy thought and he just dumped her right away since she didnt "break" like how he had initially thought and hoped she would. and so our beta MC is going to teach her about how she should take care of herself, the importance of not letting herself be touched in weird places by anyone and blah, blah, blah until she realizes something along the lines of "that she has been tainted", "that she isnt good for him" and many other stupid shit that girls (in these types of mangas) tend to think before making sure with their boyfriend or love interest and do something stupid that they will later regret.