Masculinity and Femininity are not opposites. They are complimentary inversions of each other.
If we define masculinity as being good at being a man, then a man who does so poorly is not automatically, for any reason, good at being a woman and therefore it would be sophomoric to insult women by saying that he is "feminine."
He is a coward. He is weak. He is dishonorable.
The same goes for women. A woman who is fails to be femanine will never be good at being a man and should not be called masculine.
The insult is not worth the subconscious confusion it causes.
#masculinity