Why?
The Marine Corps worked this through with a bunch of woman some time back. It's not that woman don't have the guts, heart, endurance, smarts or attitude. The problem is that this sort of demand simply destroys women physically faster, much faster, than men. It DESTROYS men. Why have women trained up this way into a job that is far more likely to make them physically unable to perform their duties much sooner than guys? I read this like "Women graduate plow horse school!" or "Women able to carry 100 pounds of stuff long past the point where their knees are ruined and their back will give them trouble for the rest of their lives"
Why?
Actually, women have several advantages over men. While men are generally stronger and in *some* circumstance shave higher endurance because of their higher Vo2 max and larger blood pumping hearts...women do have some distinct advantages in certain circumstances. Men do not always have the edge in physical activity.
Women appear to burn more fat and less carbohydrate than men during endurance exercise (Tate & Holtz, Canadian Journal of Applied Physiology 23: 570-82, 1998; Carter et al., American Journal of Physiology 280: E898-907, 2001). This offers women the possibility that, in events taking two hours or more to complete, their supply of liver and muscle glycogen (a storage form of carbohydrate) will outlast that of men. The reasons for this greater reliance on fat are not fully understood but may relate to estrogen's effects on metabolism, since male rats given estrogen burn less glycogen than male control rats during prolonged treadmill exercise (Kendrick & Ellis, Journal of Applied Physiology 71: 1694-9, 1991; Rooney et al., Journal of Applied Physiology 75: 1502-6, 1993), although similar studies in humans have yielded mixed results (Tarnopolsky et al., International Journal of Sports Medicine 22: 175-80, 2001; Carter et al., Journal of Applied Physiology 90: 139-46, 2001).
As a side note, estrogen may also protect muscles from exercise-induced damage under some circumstances (Tiidus, Canadian Journal of Applied Physiology 25: 274-87, 2000).
It has been shown that women can sometimes finish ultramarathons in times similar to those of men who can beat them in "short" (26.2-mile) marathons (Bam et al., Medicine and Science in Sports and Exercise 29: 244-7, 1997). And when men and women with equivalent marathon times are pitted against each other in ultras, the women tend to win (Speechly et al., Medicine and Science in Sports and Exercise 28: 359-65, 1996).
Also of note that women tend to perform better than when the temperature is high, as they are generally smaller