I somehow think it's more a case of "you can try but you'll be laughed right out and no one will let you in" rather than "it's FORBIDDEN". Women have generally been far more conscious about open discrimination towards them than men. That's why I didn't like the Disney treatment, where it was "women are forbidden from the army and will be killed if they join" - a good soldier is a good soldier; the gender isn't an issue, it's the lying which makes a difference. There're a lot of wuxia and wushu TV series and films in which women are very good fighters.