Just in case
This sex party takes place in total darkness, so:
People that have siblings should set up some kind of a call out code in advance... If you are concerned because this is a 'family affair' (and you know your sister/brother/first cousin/dad will be there) you may want to choose a "code word" that you could say before engaging in any activity. If you hear the code word, you know to move along. (Bananaslug, kumquat, or something like that is a good codeword).
Yeah, you could do that. Or you could just fail to utter the code word. So your dad can say "bananaslug" all he wants to, but if no one answers back, then he'll never know, right?
The possibilities for comedy in this are enormous. First of all you've got a room full of people -- supposedly carefully 50-50 boys and girls, though God knows in San Francisco that's harder than it sounds -- in total darkness, saying "Banana slug" and "kumquat" and "defribrilator" into the darkness.
"Penguin."
"Tar paper."
"Barbara Bush."
"What? Your code word is 'Barbara Bush'?"
"Hey, if you don't like it, then don't have safe sex with me."
"Hey, everybody! His code word is 'Barbara Bush'!"
"Shut up, man!"
Second, the party "will mostly be established couples looking to expand their sexual experience." Let's think that through. Dick and Jane go to the party, arriving with Don and Judy, Rob and Laura, Shlomo and Becca. Why do they all want to have sex in total darkness? I'm trying to think of the reasons. Let's say he wants to have sex with two girls at once. Or he wants to suck cock. Or he wants to watch her do it with a girl.
Okay, fine. Now it's six hours later and they're in a cab on the way home, discussing their adventures. Unless they were tethered to each other the whole time, there's no way to verify what either one did.
"Dick dear, did you finally suck a cock, like we discussed?"
"Oh, yes, Jane, and what an experience it was!" (Totally lying.) "I completely see what you mean now about the gagging thing. I will be much more careful about that from now on! And how about you? Did you manage to get it in both ends, like you said you wanted to?"
"Oh, yes! It was heaven!" (Totally lying) "I wish you had been able to see me...."
But this is my favorite guideline: CRAWLING IS SAFER.