That's all well and good that you disagree, but SF has 13.4% persons under 18 compared to the national average of 22.6% [0]. There are very few kids / families.
Agree that good community doesn't necessarily equal large community.
I live in Boston, which I believe is somewhere around 17%, and in the central neighborhoods I lived in, percentage of households with families was < 10%. Despite being small, it was a "good" community in that you got to know many of the other families in the neighborhood, I coached local sports, etc. It didn't feel small because of the density of the area AND people were generally on foot or transit and not driving. Running into a family we knew was a daily occurrence. All of those things contributed to making a small population feel much better.
You say something mathematically precise and then switch into a vague "very few". Take some walks around the parks and go people watching, you will get a better feel for what that "very few" can mean.
I was restating the line you quoted, and yes precisely SF has very few families compared to other cities in the US. A sister comment seems to indicate that even NYC is 24% (I haven't verified this.) So even for similar urbanization levels SF has very few families.
My point isn't about in comparison to other cities, it is more, does a smaller rate really mean "few" and how much of a problem is it really. It's not the problem that the most negative naysayers will claim. Note I didn't say something like "affordability poses no problems for families". Because it does. But there is indeed a vibrant community.
Census is dated. I can vouch there are a lot more families now in the city. Hospital baby birthing rooms are filling up - there is a legit surge of babies in the city right now.
The crowd arguing with me might say that a bunch of those babies will move to the east bay or similar by school age. And many will. But it's still a very good place to raise kids and grow up.
I am raising my kids here and disagree with your assessment. There is a pretty good community of families here.