Multiple factors:
1. Massive decline in birthrate in area because of reduction in breeding couples.
2. Annoyed citizens taking their families, moving to free states.
3. Net loss of population - not enough people moving to SF to replace people leaving, most people moving to SF are gay and won't have children.
4. Given the stupidly high prices in SF/CA, would *you* want to have children and the expense that goes along with it? Far easier to just use birth control.
Re: my number 4 reason - I'm not talking about abortion, just contraceptives. So, please, no fire storm. :-P