Typically I offer one of two answers.

(1) Women are a set of all people assigned women at birth minus the people who transitioned to a different gender plus the people who transitioned into the female gender but were not assigned women at birth.

Their usual accusation is that it is 'circular' (they of course have no idea of what 'circular definitions' are. Or it is to ask 'well, how do women know they are women, based on what criteria'. Even though you need no criteria to define a set - a set is just a collection of objects, in this case individuals. So I add a time dimension:

(2) At any given point in time (call it t), let's assume women are some group of people. Then at any later point in time (t+delta), women are that group which observed the group of women at time t and felt they belong to it. So, you explain gender identity via a sense of belonging to an existing group.