I think this matter is quite
simple.
There is
no difference between men and women in term of cognitive abilities or behaviour, there
never was and
never will, women can be as brutal as men and men can be as soft as women (just show a man a cat picture, they, including me, aaww like 12yo kids), but there is a little natural difference in term of physique, but this takes effect AFTER the puberty and even then, who cares?
Sometimes you can't tell a boy apart from a girl at this young age, they are pretty much the same, except both have different kick-in hormones, testerones and estrogen and still both genders have them, but boys have more testerones, girls have more estrogen/less testerones (don't trust me on this one, not a doctor, but i think i read it somewhere

). Ever wondered why you have nipples as a man? Because your body didn't know at this point of development what you are.
At this young age neither a boy nor a girl is better at anything, they are the same, they treat themselves the same, they are just children, they don't distinguish between genders, they distinguish between friends, foes and family.
WE, the society, tell them WHO they are, even if they
don't want:
You are boy? Wear this blue shirt, take this car toy and this action figure. You want a doll rather than this car? Don't behave like a sissy!
You are girl? Wear this red shirt, take this little house and this barbie doll. You want the car rather than the doll? Don't behave like a tomboy!
So in the end this is not a "natural" decision. But not quite time ago it was normal to have male soldiers and not female soldiers, why?
Well maybe women didn't want to be a soldier or maybe the
society didn't want them, both is and can be true, still today.
But the main fact why there aren't any female Witcher is probably, because Sapokowski didn't want them. It fits the middle age setting to have primarily male soldiers and not female soldiers, still there are some characters like Rayla or Milva, who are indeed badass fighter and most men didn't want to encounter her and furthermore they were respected by their male fellows and if they weren't, well, ouch

And don't forget there is one "kind" of female soldiers in form of sorceresses, sure they don't fight with a sword, but rather with their intelligence, but still they are a kind of soldier in this (political) war and don't forget sorceresses invented the witchers.
And at last the mutagens, as many have already said, maybe the mutagens are designed to change boys and can't interact with the female physique or hormones, we don't really know. When Ciri was at Kaer Morhen, the other Witchers didn't want to give them to Ciri without knowing what could happen, so obviously there was never a case of a girl wanting to be a witcher, most girls probably want to be a sorceresses and this is again a society effect.
If the witchers at Kaer Morhen were cruel and emotionless they could just experiment on their own and why not use a girl? what is the worst that could happen? She dies? Okay, next one.
So there is not really a NO-GO argument to have not female witchers except witchers are not longer needed, so it isn't a desirable profession (it probably never was), nobody wants to be a witcher, an outlaw and presumed criminal. They want to be powerful sorceresses, who can affect a whole kingdom with a flick and we all know they can.
There is not a even slightly reason why there shouldn't be females witchers except there aren't any and that's it. Just because there isn't it, doesn't mean it can't be.