Men locked in Ukraine

The definition of Civilian in international law is "not military". That does not mean, however, that the government cannot force some form of service in times of emergency even amongst those not in the military. Or even non-emergency, as many European countries had, and some still have, the mandatory choice between military service and civil/community service, the latter are still civilian even though it was not voluntary.

Arguments over sexual equality are good in academic circumstances, but the practical reality is that some form of simple dividing line is required which can be easily processed by the executive organs. The country cannot keep tabs of every citizen's personal circumstance to determine if they're single parents, working mom vs stay-at-home-dad, age of offspring, dependent caregivers, or anything else. Making the dividing line "able-bodied Men of military age" vs "everyone else" is the simplest form of distinguisher which will have the most efficient result in manpower capability.