This one appears to have hookers available as well, but the one in my city is actually a very clean and elegant place for a date. Depending on your city there may be something similar. And as everyone else has already said, if you just wanna hang out naked and do spa things, search for Korean spas, Russian banyas, or Turkish bathhouses. I went to one recently and -wow- awesome. Anyway here's an article about it. In fact, between and early last decade there were both gay and lesbian bars, show bars and bath houses in San Mateo County. For the second part, if it exists, it probably isn't publicized - so you may need to find a crowd that is into that sort of thing first.
If you want to have sex in public, find somewhere that has public play spaces.
On Tuesday, this de facto gay sex club hosts "naked nights" with complimentary clothes check. And pretty heavy on queer and BDSM culture. There used to be a place in San Diego that was kind of similar, but more like a hotel where you rent a room with a hot tub for a while for your own private thing rather than being a place to meet new partners.
In fact, between and early last decade there were both gay and lesbian bars, show bars and bath houses in San Mateo County.