In addition to the whole Y chromosome thing, I have discovered a key difference between men and women. This observation was first made while I was out at a bar with a male friend, and a guy standing near us pulled off his sweatshirt.
Male Friend: Look! That guy just took his shirt off the girl way!
Me: The girl way?
Male Friend: You know, the sexy way!
Me: The sexy way? What are you talking about?
Male Friend: You know, grabbing it from the bottom front and flipping it up over your face.
Me: That's not the girl way; that's the normal way.
Male Friend: No, it's not. The normal way is grabbing the back of the collar and pulling it forward over your head.
Thus, I became aware of the gender differential involved in shirt removal. It seems to be fairly consistent: girls remove shirts from the front, pulling them backwards, whereas guys grab their shirts from the back and pull them forwards over their heads. Which seems like a very cumbersome way to remove a shirt, if you ask me.