I think what @Arkandel is talking about can more easily be demonstrated by weightlifting: There are Men and Women's divisions for weightlifting because there are some pretty basic differences in the average-to-high end of physical strength that give men an advantage that can't really be matched in terms of frame size and how much muscle can be stacked on it.
The women who compete in these things are not wilting flowers by any stretch. They are just as dedicated, just as ridiculously ripped, just as focused on proving themselves, but the men's records are anywhere from half again as much (for comparable weight class lifters, i.e. these people all weigh in at X) to three times as much at the high end (men have 4 full weight classes above the top female weight class in world competitions). The heaviest female weight class for lifting barely tops the lightest male class for lifting.
Were you to say that it's somehow sexist to make 'the girls' have 'their own little league' and they should be able to compete with the men on equal terms, they would basically never place. It would effectively be like letting heavy weight class lifters compete directly against light weight class lifters, all across the board.
DISCLAIMER: None of the above is meant to imply that women shouldn't serve in combat, or aren't just as capable of being soldiers or at all manner of athletics and etc. I don't believe that to be the case. It is the case, though, that sometimes physiology plays a distinct part in giving advantages to one gender over another, and people ignoring that in the name of pushing 'equality' is, I think, what @Arkandel was getting at. I could be wrong.