A friend of mine, who was an instructor Marine, gave me what is probably the best reason I have heard for not allowing women into some jobs in the military. When he stated that he didn't think women should be infantry, I (and several of my friends) laid into him, asking if it was because they screwed up the dynamic? He said no. Was it because they caused trouble with the men? Nope. Because they're too maternal? Hell, no, he said. Probably three of the best five Marines he had handled in boot were female. Was it because they weren't as strong as men in the upper body? No, he said, noting that women had much better endurance than men, and the times when that upper body strength was a problem vs. when endurance was valuable made that laughable.

Okay, we said, stumped so far. Moral reasons? No way. He was adamant that if men fought for their country, women should/could too. Finally we gave in, and he told us.

When he was training a mixed platoon, the women had, in many cases, outperformed the men. He noted that he didn't think the selection effect of these being the more 'driven' women was a factor, here; he just plain thought they kicked ass. However, after several weeks of side-by-side training with the men, in mixed companies, several of the women began lagging behind in marches and having difficulty on the obstacle course. None of them had complained, and none had taken any measures other than Tylenol to keep up. He and several of his colleagues, who were all noticing this problem in their platoons, had a full medical check done on all boots, both male and female.

Around 85% of the females had multiple stress fractures of the pelvis. This was why they had been lagging. None of these were due to trauma injury; all were due to the fact that (according to the surgeons and orthopaedic specialists who looked at the data) the female pelvis simply wasn't designed to handle the loads placed on it in a normal training regimen, most specifically in terms of lugging weight (critical for infantry).

This was why, in his opinion, women shouldn't be in the infantry. Simply because the normal training regimen caused bone damage. He was of the opinion, and I agreed, that if you had to modify the training in any way (other than say having separate bathrooms) in order to get women through, then you had a severe problem.