It's about the physical force inherent in a bipedal hominid. It is true that females are smaller, which is why I sort of agree with Titan that not only are they the fairer sex, but that they should be treated as such in an almost reverent way. But if an individual female chooses to step out from under that umbrella, then she will very likely get rained on.
Men are not the ones who fight in wars because of social construction but rather because of Darwinian necessity. Women do not go through labor (and often die therein prior to moden medicine) and obsess about babies and nest-making due to social construction but due to Darwinian necessity.
Most men, as boys, play guns and swords because they have an instinctual knowledge that physical combat both can and likely will be the greatest challenge of their lives and that the ability to succeed in such an event is directly tied to their ability to pass on their genes. Women, as girls, play with dolls because they have an instinctual understanding that rearing a completely helpless homo-sapien, complete with its gigantic brain, requires a colossal effort and that it is directly tied to their ability to pass on their genes.
This is not to say that a woman is incapable of causing damage to a man however. She is smaller, but only somewhat smaller. She is still much larger than say a dog or a wolverine. Evolutionary speaking however, she is not adapted ( physiologically or psychologically) to combat when compared to males or sharks and thus should not be surprised when she fails at it when compared to males. Sadris can shoot as many poz loads into as many man-asses as he wants but he will not succeed in passing on his genes. A woman can initiate combat as much as she wants but this behavior will likely not succeed in passing on her genes.
If a woman wishes to initiate combat of any sort, and be expected to be treated differently based on her gamete profile, then she is the one being a social constructionist and is the one who is completely disregarding her biological imperative.
It's not exactly that she is incapable of hurting a man, it is that she should know what she is truly up against, in a Darwinian sense, when she chooses to attempt to do so.
It's an interesting topic. I see it as one of the lines between second and third wave feminism. The second wavers would say something like, "Women should be allowed in combat, but they should also be aware of what they are up against." The third wavers would say something like, "Women should be allowed in combat, but with special 'kid-glove' rules that apply only to them."
So for me this boils down to the concept that I do understand the gender roles given to us by evolution, and I appreciate them, and I do tend to treat women with a lighter touch in all aspects for this very reason. Not only do they require it, they deserve it. However, a hominid mammal who initiates combat should expect to lose that combat unless they approach it with the utmost respect for objective physics. The minute they inject subjective social construction into objective physics they should expect to hit the pavement.
The rising and setting of the sun are not social constructs any more than the biological nature of sexual reproduction, the competition for resources, and the roles that each sex plays in that balance of genetic survival.
This is also why we tend to hate on men that initiate combat with women by the way. Think on that Vanessa. When a man hits a woman, most people denigrate and/or arrest him... and rightly so. But this is also the same exact reason why a woman should not hit a man. Do I really care what gender that Kodiak Bear or that Dog is that tried to bite me? No, I simply shoot the damn thing and ensure that I live to make it home and care for my genetic partner and my genetic offspring. And all of this is only my biological reasoning. Philosophically there are many more arguments to be made about self defense and rational self interest.