I don't care what sex you are; I don't feel (and didn't in high school, either) comfortable dressing and undressing in front of others.  Perhaps the schools now need to find it in their budgets to build facilities that are more private.  (Ack!)  One of the reasons I enjoyed taking riflery in high school was because I didn't have to dress out and still got my gym credit.  I'm just extremely shy about such things; I don't think I'm alone that way, either.