Are females more flexible than males in all mammals?
Try birthing a child for once. You need to be flexible to accommodate the growth of another organism inside you. 

Take a look at some pictures of the mothers spine during the course of a pregnancy, you will have no further questions. Same with belly size, breast size and vagina size upon birth.

This video is a nice explanation of this. Also includes the different centers of gravity for males and females (human).

http://www.youtube.com/watch?v=WO8DtsZO1nA