I'm going to have to agree with what has already been said. I think your roommate simply has a very limited misunderstanding of what a true D/s relationship is. In my opinion when someone submits to me it becomes my responsibility and desire to protect them. I would suggest offering an explanation to your roommate of what true D/s dynamics are about...it seems to me she simply has never had the opportunity to have common societal misconceptions cleared up for her.