If you’re opting for something that’s as realistic (in a fantasy-based context) as possible, that’s probably what you’ll go for. A while back I did some groundwork for a military fantasy (think 300 meets The Black Company) and it had almost no female cast members at all. [For those wondering, I decided to go with my present, and almost finished, book instead of pursuing The 300 Company].
Modern women are told, rightly, that they are quite capable of doing the various things chaps can do, such as leading large companies, running countries and committing heinous crimes. However, it was not always so. The only woman I can remember being mentioned in the entire Second Punic War, which lasted for decades, was Sophonisba, and that was because she slept with two Numidian kings and then got executed.
Fantasy is often rooted in history, but it is the case that throughout time there have been occasional and glorious women who have forced their way out of the home and into politics or onto the battlefield. A very early example was that in the navy of Xerxes there was a female ship captain, who was his particular favourite. The rebellious (though ultimately unsuccessful) Boudicca in Ancient Britain is another fine example, as is Zenobia, the ruler of the Palmyrene Empire who had the misfortune to face Aurelian.
Powerful women also played a significant role in the Byzantine Empire. Unusually, the wives of the emperors had a real, defined political role, almost like the First Lady of the United States, but in an imperial setting. Due to the surprisingly high number of very rubbish Byzantine emperors they quite often played significant roles helping the empire when the nominal man in charge wasn’t up to it. It’s worth mentioning also that quite a few of them were a bit, er, frisky and sometimes meddlesome, as the wife of Justinian the Great was.
The reasoning behind the lack of female warriors is, I would argue, more to do with demographics than anything else. Until quite recently childbirth was very, very dangerous, and huge numbers of women died giving birth. Because the number of women define the potential size of the next generation more than the number of men (a man can have, theoretically, thousands upon thousands of children, women cannot, especially when childbirth is risky) it as vital for the future prosperity of a city or nation that women survive. In short, men were more expendable.
It’s also true that, generally, women are weaker than men (not just in raw strength but also in terms of resilience when bearing a heavy load for a long march).
Of course, the fantasy writer does have an easy get out clause if he or she wants to have plenty of lady warriors. A race of Amazons solves the problem at a stroke, and women can also be put into positions of spiritual or arcane power.