In SF, there really aren't any real "bathhouses" due to an old ordinances. They are mostly sex clubs. There is eros which is the most like a bathhouse (ie sauna, steamroom, towel dresscode, etc) but it does not have private rooms nor do they have an area to douche. The facility that best fits the traditional bathhouse definition would be Steamworks in Berkeley. Which is a 15min car ride from the city or you can take BART or a cab but traffic can be unpredictable and the cab ride could be around 40 dollars or more. They sexclubs are the Brig and Blowbuddies. That's pretty much it. There are the arcades w/gloryholes but again no bathrooms. Best of luck. Unfortunately, compared to Toronto, Montreal, Chicago, or even LA. San Francisco is pretty limited with the traditional sex venues. However, they are SERIOUSLY liberal with cruisey backrooms and play night at certain bars (Powerhouse (any night), Truck (Tuesday nights), and sometimes the Eagle (special Saturdays). I have seen guys getting fucked bareback and it at these venues and it's not an uncommon sight.
Also, it's a pretty gender segregated town. Normally, if chicks are around the dicks stay down. However, try PowerExchange (different from Powerhouse) which is VERY trans and sometimes bi. Have fun.