I think that this argument about kids in San Francisco is totally wrong. There are tons of kids in San Francisco...just not in the neighborhood that he went! Go to the Richmond, the Sunset, the Marina, Cole Valley, or especially Noe Valley...and you will see tons of kids. But sure, in SOMA or Union Square/Financial District...not so much.
Why is that ? Because San Francisco is a very neighborhood-focused city - where kids stay in kid friendly neighborhoods, and where younger hipsters or developers live in more fun neighborhoods with typically less babies.