A few things.
Regarding women in the military:
70 pounds of gear is not a lot. It sounds like a lot, but the weight of military gear is designed to be very well distributed and much more comfortable to carry across long distances than civilian gear. That's about the weight of your flak, your kevlar, a light pack, an M16, water, and few magazines. So if you're moving any significant distance in the field, you will be carrying at least that. That's not a weight applied to special forces. Even marine recruits in boot camp carry that much on their first military hikes. And female recruits go through almost identical training. The only difference is raw strength tests. In terms of tests of military proficiency, like hiking, marksmanship, etc., women have to rate just as high as men. And last time I heard, roughly 1/6th of the marine corps is female, so there's a significant number of gals doing that and succeeding.
That's actually one really frustrating thing. People think of the military as super conservative. In reality, the brass is, but the people on the ground are usually way ahead of American society. The brass were still bitching about how gays and women were bad for combat efficiency probably 10-15 years after the average enlisted person stopped giving a ->-bleeped-<-. I knew this obviously gay guy in boot camp, who happened to also have the worst name possible for a gay recruit in boot camp. Recruit Cox. Guess who gave a ->-bleeped-<-, out of any of the recruits or drill instructors? Nobody. That was the norm even then. The military itself, with the exception of a few generals born in the year 1900, and a few homophobic ->-bleeped-<-s who made a nightmare out of the policy, had basically thrown out don't ask, don't tell 10 years before the civilian world was kind enough to save us from it. Hard conditions force you to forget about a lot of your bull->-bleeped-<- bias and respect raw ability.
Most countries which allow females to serve as infantry haven't seen any huge decrease in combat effectiveness or any enormous logistic concerns. Canada does it. And before you write off the Canadians as a military force, their troops have seen heavy combat in almost all major wars in the United States has participated in during the 20th and 21st century. They had their own beach to storm on D-Day, and it was almost as bad as Omaha.
Even the US military already has women trained specifically for combat roles. They're not technically grunts, but they have similar training. For example, military chaplains of various stripes are not allowed to carry or use weapons. This is to preserve their noncombatant status. They usually have a theater appropriate escort when traveling, but beyond this, they are assigned one specific staff member whose job it is to follow them and protect them at all times (even in the US). I've met women in this role. It's straight-up combat training. They have all kinds of close quarters training, as well as proficiency in a dozen weapon systems. And they do those jobs just fine.
Regarding the "biological basis" of female behavior, particularly parental instinct:
We don't know every factor that drives that, so researchers typically focus on a few. A good example is the hormone oxytocin. This is typically a considered a female hormone, and it's considered to be a strong driver of "maternal instinct." In women, its production is triggered by things like childbirth and breast feeding, so it was simply assumed that men did not have any significant quantity of it. This was an incorrect assumption. Recent research has shown that the mere act of playing with and having direct contact with their children dramatically increases the level of this hormone in men, to the point that some men who interact frequently with their children have higher levels than their wives who have recently given birth.
Recent human sexuality researched has shown that many of the things we assumed to be biologically sex-based were really just a mild gender disposition for that behavior which was reinforced by social norms.
Even aggression. We assumed higher testosterone equals higher aggression. Not we see things differently. We know testosterone acts in concert with other hormones, that it alone doesn't lead to higher measured ratings of aggression. We also know that the link between testosterone and aggression is as much cause as effect. Women measured after highly competitive activities (athletes in sport) or after receiving a provocation in a controlled setting show levels of testosterone above their own normal level. And those who also have elevated levels of certain other hormones typically measure high on ratings of aggression. So sometimes competitive or provocative activity itself causes elevated levels high enough to cause aggressive behavior, so baseline levels are less important. And we also see that it's less about raw quantity and more about the percentage increase above the baseline for that biological sex. Highly aggressive women can get there with a lot less of an increase in testosterone.
