Women can't easily be conscripted - the beaucracy involved in issuing exemptions for women at a time of crisis would be extraordinary. Whats most needed is for the vulnerable to go (and be helped to go rather than be a burden) and the most phyiscally able to stay and fight. In practical terms this is simply the easiest way to do that, rather than making lengthy assessments no one has time for.
There is also basic things like bullet proof vests designed for male bodies - they are not suitable for women. That means women are more at risk. And stuff about the weight of military equipment. I couldn't physically carry it. Some women could, but most men can. Ironically, if all your weapons and protective gear are designed for men's bodies, you've already had the decision made for you in some ways...
There are plenty of roles that they can be conscripted to as support, and are being encourage to take, but the fact is they are inherently less suited to front line roles than men. There is the extra physical risk of women - rape is a women of war. Its important to understand this; including the impact on males on the same side. This doesn't mean women shouldn't fight. There are lots of women who are taking their children to safety across the border and then RETURNING to Ukraine. Or sending children with other relatives.
We can't escape this as a reality anymore than biological reality that women give birth.
This isn't about gender roles its about sex and biology. The two things are different and the conflation of the two isn't helpful. We need to understand and appreciate the difference and where it doesn't matter and where its an unavoidability reality that really disadvantages one sex or the other. Where are women capable? Where is sex holding them back unnecessarily? Where is sex an active issue?
Eg: Health care outcomes for men and women should be equal, but we do have different bodies which mean our symptoms are different, but investment in women's health is disportionately lower and less is understood about women's bodies.
Therefore all bodies are capable of giving us a good quality of life.
But the differences mean we have different needs.
Women are being underserved because of social prejudges not because of physical deficiencies.
At a time of sudden crisis, the rule makes sense, otherwise you'd generate a lot more problems just when you need as few as possible.
As it goes, the UN are reporting that 2 million have fled Ukraine so far. Of these 1 million are children. Many of the remainder will be elderly. That really does point to a hell of a lot of women staying too though.