[T]he criticisms of women in combat today remain based on the doctrinal assumptions of yesterday. These criticisms are totally anachronistic and divorced from contemporary reality. It would be literally impossible to pull women out of harm's way, because of the way the battlefield has changed. Every unit goes into harm's way now, not just the infantrymen or the tankers or the Green Berets. If you wanted to reinstate the "risk rule" that Secretary of Defense Les Aspin killed in 1994, you'd have to pull women out of the entire Army — or at least ensure they never deploy anywhere.
The reality is actually far more complicated than even this. Bluntly, there has been no true "rear area" since the advent of strategic bombing. That means men, women, and children; soldiers and civilians; everyone. Just ask anyone who lived throught the Second Thirty Years' War in or near Dresden, or Köln, or Tokyo, or Coventry, or … Even less was there a true "rear area" during the era of strategic-nuclear standoff from 1950 to 1990 (roughly). Perhaps this is more evident to me from an Air Force background than it is to someone from an Army background; but, in the end, I believe that Mr Carter understates the point more than anything else.
In the best of all possible worlds, we would not have to argue about this. We wouldn't be putting anybody's children in harm's way in the interest of national security. But that's not even utopia; that's a delusional glimpse at impossibility. So long as we're putting anyone in harm's way, it should be those best able to perform the mission. The last time I checked, "survival instinct" was not gender-selective.
Mr Carter's other point—that putting the women into frontline units might increase the risk of sexual harassment and misconduct—is, in my experience, not well founded. (Note that he does not consider this risk sufficient justification to pull women from those jobs, though.) While working with deployable air units, I noted substantially more misconduct of all natures revolving around sex when at the home station, rather than on deployment. And the closer that deployment came to operational conditions, the better the unit worked together—even in shielding female NCOs and airmen from predatory fighter pilots. I also noticed that post-deployment, it took a long time before true misbehavior returned to predeployment levels. So, if anything, that argues for tighter integration of men and women in the military to limit these problems. I suspect that some of it was trading one kind of pressure for another; but, at least in terms of unit morale, it was a good tradeoff.
When President Truman desegregated the armed forces by executive order, there was an adjustment period. Some would argue that the adjustment period lasted into the 1980s… but the point is that the military was stronger even during that adjustment period than it had been before. I see no reason to believe that things should be different for women; or for gays; or for any other "group" the membership in which does not directly impact the ability to do the tasks required of any soldier, sailor, or airman. One can even argue that continued segregation reduces military preparedness by simultaneously "tenuring" less-capable individuals in critical jobs and excluding more-capable individuals from those same jobs. But far be it for me to suggest that rationality should rule military personnel policy, since it never has (just look at the class profiles at the military academies, and reflect that this is a huge improvement over even two decades ago).