> In the one year I've been to SF, I've ran into butt-naked people numerous times (in contexts ranging from them just walking down the street to them running the Bay-to-Breakers marathon).

During B2B a couple years ago I actually asked an officer about that. Apparently it's 100% legal to be naked in SF, as long as you're not being too sexual... I don't know what the actual line is legally, but that's what I was told.