"No one questions why there aren't any females in the NFL, NBA, MLB, NHL, etc. Olympic athletes are the elite of the elite. No one questions why the women compete against women and men against men. Those are great sports and achievements. But lives and missions aren't on the line. In our world, if you move slower one day, you don't get bumped off the medal stand, you could die or get someone else killed."
-An anonymous female US Marine from this excellent article.
See also this outstanding related article.