National armies are traditionally male-dominated, and still remain a hostile environment for women, even those who have volunteered to serve in them. No country has succeeded in creating a gender-neutral or non-discriminatory army, and efforts to do so are a waste of time.
A separate army for women is an effective response, to that ingrained historical character of the military. However, that would threaten the whole idea that every nation-state has its own national army. That is true for any autonomous military force, not just for a women’s army. It does not matter which group gets an army or a militia – cyclists, bakers, farmers – they all present a threat of insurrection.
On the other hand, it would be perfectly logical for a women’s state to have a women’s army. In that case, there would be a match between the nature of the state, and the nature of its armed forces. So the issue here is not the ‘state monopoly of force’, but rather the fact that the nation-state does not live up to is own pretensions.
An ideal nation-state is by definition inclusive of the nation and its members. That ought to be reflected in balanced and representative armies, and parliaments, and governments. In reality, no national institutions are staffed or controlled by a cross-section of the population. Sometimes that is functional – the army can’t recruit babies anyway. It might also reflect traditional attitudes to functionality, in this case the belief that ‘women can’t fight’. Mostly however, it simply reflects structural inequalities in society, which contradict the fiction of national unity and uniformity.
The ‘state monopoly of force’ can not justify the maintenance of such structural inequalities. Where institutions are beyond reform, such as the army and the security services in western societies, parallel institutions are the best option. The fact that they may ultimately result in a parallel state, and the collapse of the nation-state, is no reason to reject that option.