As a Finnish person this question makes no sense. Everybody goes to the sauna. The young, old, babies, the sick as well. I go to the sauna even with fever as it feels good with the chills.
There is no such thing as a standalone sauna that's not also a pickup spot, though would love to be proven wrong.
Archimedes Banya (www.banyasf.com/) is my favorite. They have a real Finnish sauna that gets up to 100 C (or over) along with a pool for cooling off that is less than 10 C. They also have two Russian baths and a Turkish bath (steam room). To get that real Finnish/Russian experience, you can even bring your own alcohol. Clothing is optional on one half and required on the other.
There are others in SF, but I haven't been to them.