Exists for race, sex, transgenders, martial status, disability, careers, sexual preference, age, and HIV status. Given it's the sex party, that sound like it's probably designed to combat discrimination against prostitutes etc due to their profession.
The problem is not one of discrimination, it's just for whatever reason far less females choose to participate in party politics. If say 80% of your active membership is male then that is no doubt going to be reflected in preselections. It's definitely a problem of participation, which the proposed policy will do nothing to address.