Keira Knightley explains why women were considered the weaker sex: It's due to the corsets.
The actress had this revelation whilst acting for movie "The Duchess".
For the role darling Keira had to wear many corsets and while some of them fit very well, "some of them I could breathe in, and some of them I absolutely couldn't... and it was a complete nightmare!" Her conclusion was "It's obvious why women were called the weaker sex, because you can hardly breathe in a corset. It's a fight just to get oxygen to your brain."
Thank you, Keira. That explains everything.