I know... abortion really has nothing to do with it. SF has the most expensive housing market in the nation, and schools aren't that amazing, so any middle-class family is going to move somewhere in the suburbs where they can buy more space and send their kids to good schools.
It's happening in cities everywhere as they prosper and gentrify. Poor families leave, middle-class people stay until their kids get to school age and then they flee for the suburbs. What we have here is an efficient allocation of resources where cities and suburbs serve different populations.
I know. I really hope to live in NYC for the rest of my life (flame away freepers!). Nevertheless, I know that if my future wife decides to drop one, I'll be moving to some strip mall hell.