Female prisoners are incredibly vulnerable; it's cruel and unusual punishment to incarcerate them with males.
Transmen are unlikely to choose to be housed in the male estate, and if they do, are going to have a very difficult time of it in prison.
So this is a law that is designed to prioritise males. Males who wish to be incarcerated with women.
California has lost it's mind, along with any progressive credentials it might wish to claim. It is not progressive to force women to be incarcerated with men. It's deep misogyny.