Anti slut defence is a social condition imposed on girls through society. Men are heroes for shagging loads of girls, girls are sluts. No girl want the peer disapproval of being a slut, so she has anti slut defence to make sure she does not appear as a slut to her peers.
This is why winning over her friends in a set is just as important as winning over her. There is no real reason for ASD except what is force fed a girl through media, friends and our culture.
At least that's my take on it, I have not done any scientific research on it, and anything I say is just my common sense, flawed as it might be

(If I missed anything in this disclaimer take it up with the complaints department)