First, you have to consider the issue of logistics - if women are, say, 10% less effective, on average, than male recruits, then you are essentially dragging around 9 troops for every 10 mouths. That adds up, especially when running logistics is one of the most challenging parts of pre-modern warfare.
Second, you do have to consider the issue of sexual violence performed upon the minority gender (ie women in this case) disrupting cohesion. It's difficult to trust your rapist, or people who side with him. Also one of the reasons Russia's military is so dysfunctional, though in Russia's case it's male-on-male rape.
Third, you have to consider the possibility of pregnancy. Soldiers fuck. Each other and locals, it's unavoidable. In a time before reliable birth control, a woman who ends up pregnant has her logistics footprint increased AND her combat efficacy decreased.
Fourth, you have to consider that most pre-modern armies were not standing, professional armies. There wasn't strict criteria to get in, it was "Your city-state/village/fief is getting called to arms, round up your warriors and send them to the ad-hoc army that's forming". As such, it would be difficult to ensure that the women who arrive meet that 10% standard.
Different societies with traditions of female warriors resolve the issue in various ways. In the raiding warfare of the Vikings and Celts, there is widespread evidence for women warriors, because 20 lbs and less dense bones is a disadvantage in individual or small scale combat, but far from an insurmountable one. If you were fierce and eager, that was enough. This is also why female pirates pop up surprisingly often for a profession that involves being stuck on a ship with a bunch of sailors - personal prowess counts for more in boarding actions than mass. Many steppe tribes also have plentiful evidence for women warriors, and this is likely back to the mass argument - namely, that when you're on top of a 1000 pound horse, 20 lbs either way is not gonna make much of a difference.
Some societies, like the Sengoku-era Japanese, trained their noblewomen in the arts of war, allowing them to act as makeshift garrison troops in a pinch - much more useful than just another mouth to feed in a siege. Because of this, and the fact that the way of the samurai is the way of the horse and bow, women also participated even in the mass field warfare of the Sengoku era, though obviously not in numbers comparable to men. Some European noblewomen were similarly trained, but it was much less common due to deeply-rooted cultural misogyny. The Sikhs also trained women in warfare and had significant female participation, but I only know the rudimentary details of that.
Obviously all of this is irrelevant in the context of modern warfare. But it's good to understand the 'whys' of widespread historical phenomena, like women being excluded from military service.