It's not about physical or mental ability, don't you guys see?!
Men serve in the military forces to protect their loved ones, their family and their country from the bad guys. It grants them a sense of purpose and mostly, great honor. When their women serve in the army, they will lose their purpose since what they protect is no longer safe. Thus their honor is robbed from them under their very feet. What is not left is a soldier, but a mere warrior with no purpose, honor or family waiting for him. He fights just because he likes to fight. What is left is a poor, poor savage.
As my ol' grandma always said: "The men serve in the military, and the women bear their children".