Great that you ask where do you draw the line. You provide an excellent example yourself.
Earlier you refer to people with green eyes. Discrimination on this basis would obviously be arbitrary and wrong (unless there's something you know about the green-eyed that I don't).
Women here are referring to on males, who commit 98%+ of sexual assault and are up to 120% stronger than females, thereby enabling this.
I don't honestly think you could get a clearer indication of absence of need for any line (your green eyes at 0% difference in risk) and justification for a line (my near-100% difference in risk).
Aston explains this beautifully in her longer post a few pages ago.
ETA: Obviously, even then, the disadvantage to men - which I can acknowledge here - means that thought and care is needed in implementing a policy like this. Hence Aston's explanation of the law - laws democratically developed on the basis of extensive data - as well as the financial imperative etc. Drawing a line requires making a judgement call. But the judgement call doesn't get any clearer than the quite frankly extreme stats above.