@Kathryn and Erin
You two hit something spot on. There is much truth to the biological, psychological, and social aspects of being a woman, and how women are treated in society.
@Kelly
Your response talks about what femininity truly is. Although there is the social, biological, and psychological aspects of being a woman, femininity is a force. It lies at the core of all women, regardless of what body you were born in.
I can totally relate to your this. I had a friend who told me when I came out how she felt a certain level of comfort with me that she only feels around women. Another female friend said that she just knew I was a woman inside a man's body all along and she was just waiting for me to come out. She just felt that I was so feminine. No other words could explain it. These two women picked up on that force of femininity radiating from within side of me. That same force that creates my gender identity. That force that others were able to pick up on.
It is that force that even when I was pretending to be a man all these years, triggered a lot of people to suspect something feminine about me, and often they thought I was gay. Even though my behavior was nothing like a gay man, and even though I passed well as a nerdy, shy man. There were a couple of personality traits that helped as well - not chasing after women and not being terribly aggressive - helped people to suspect something feminine about me. But it goes deeper. I'm not a gay man. I'm not a nerdy, shy, straight man. I passed as a nerdy, shy, straight man. But I am a woman on the inside and always have been. I am now making my outside match my inside.
It's a great feeling to have people see that woman being reflected back visually. Before I transitioned, the mismatch of my inner feminine core and my male body not only created great conflict in my life, but created conflict in how others perceive me. Now as my outside changes to match my inner feminine core, people see that and it affects how they treat me - for both good and bad.
But I believe aside from any social constructs around gender, being a woman is ultimately about being feminine at your core. It is about possessing that core female energy, your soul, your very essence of being.