They've already been force mobilizing some women as well for quite a while now (primarily those with medical training), just not en masse yet. That will probably come after the inevitable lowering of the mobilization age which the West has been heavily pressuring Ukraine to pass for almost a year now. You constantly see comments made by Western officials and articles written by Western commentators that Ukraine needs to mobilize its youth. In the meantime a lot of women have been joining essentially out of economic desperation because it's either that or their children starving on the streets after all the men around them have disappeared.