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Of all the shameless acts of social networking, sex parties are the most off-the-graph. Officially, there aren't any orgies in the Valley. None of your friends go to any, certainly not. So why were they at this private party in Marin, with a digital music pioneer as host to an epic pool sex scene? More to the point, why weren't you?

Here's how it happens: This guy has made a decent living in the music tech biz, and he and his handpicked-for-hotness friends rotate the party among a set of over-the-top houses. A complimentary driver with a discreet party van ferries 200 of us to a house just past the Golden Gate. It's swanky, with a coat check and a champagne bar. It's legal, too. No tickets required, but door donations are the norm.

I recognize a coder from the gaming industry I met at a party a few months before. That event was mostly attended by the Burning Man crowd. The women here are pure L.A.: trophy wives who do a lot of Pilates, aspiring massage therapists, a couple of supertanned strippers who retired at age 24. A few are doing lines of coke in the powder room. Many more are outside, hooking up in a hot tub built into the swimming pool.

Inside, most of the action is in a harem room: soft carpets and pillows, low light, and a welcoming atmosphere. Nearly everyone is here as part of a couple, which means lots of threesomes and foursomes. Only one thing shocks me: so many people here already know one another that condoms are very optional.

Parties like this are invite-only. To get in, you have to know someone who knows someone. I found my way in by getting to know a few semi-public sex party regulars and demonstrating why I'm a good girl to invite back. Semi-public sex parties? Those are everywhere. But that's another post.

(Photo by susanica)