I think she was looking out for her. She rightly reminded her that her mother may be watching and probably wouldn't want to hear the way she was speaking. The conversation was so contrived, Marnie was trying to be as crude as possible for attention. It wasn't relevant to the conversation they were having, so there was no reason why she should start talking about having sex, vibrators and masturbating. She has the intellectual capacity of a slug and showed no respect for the other people in the room.