The kinds of strangers you are likely to meet in SF are not to be trusted. It's not irrational on an individual level, it's just a societal madness in the USA.

I lived in LA, and would not let my wife walk around after dark let alone my daughters. There were a few individuals who lived under bridges that would regularly assault women. And we lived in a "good" area. We moved to an even better area and within a couple months there was a shooting, high speed chase, and a drunk driver rolled his car into our neighbor's yard.

Needless to say, we moved away.