I'm not going to take it that far - I think the concept has a "good faith basis" in the common sense idea of "combat requires strength, and men are stronger."
But, when you look at what's actually happened - and when you take into account that we don't fight battles a la Braveheart (and I know, there's always the chance that the Afghanis will charge the field at Bannockburn, fight like warrior poets, and we'll decide the day with claymores, heavy cavalry, and Henry-V-like speeches, and then crown a new king of Scotland, thereby achieving "freedom," even though we continue to live under the same feudalistic system as before, except, wouldn't we be the eeevil English, so we'll definitely have to keep our primary focus on brute strength instead of people with good trigger fingers and good brains) - women clearly do fine in combat in the modern era. If (in light of what Dan rightly pointed out) your focus is solely on combat effectiveness, it's either not decreased or is actually bettered.
Sorry for the parenthetical rant re: Mel Gibson's take on the Middle Ages. Which is also apparently his take on the American Revolution, which was apparently the same as the Scots' struggle against Edward I, except with flintlocks.
Rabble. Rabble rabble.