I would suggest that it is a slur because it is used as a term of disparagement that often doesn't even accurately describe the target. The label gets thrown at women who are not RF, and sometimes women who, for whatever reason (for example, a religion they adhere to) would be reluctant to even call themselves F. At that point, I don't see how the word is any different than people e.g. referring to any Arabs as "camel jockeys".