Her: You need a local girlfriend. One you have a lot of sex with. While I watch.
Me: Okay, you're on.
Her: We need to have rules, though. We need to tell her that she can sodomize you with a strap-on, but no oral sex.
Me: No oral sex? But I like oral sex!
Her: Yeah, but...um, no oral sex because...um, because your cock belongs to me. Or something.
Me: Hey, I've got an idea. Write down every sex act you can think of on a sheet of paper. Then go through the list and roll a six-sided die for each one. If you roll a 1 or a 2, she's forbidden to do it; otherwise, she can.
Her: Hey! We should do that every day.
Me: So the rules change every day and she never feels secure?
Her: Yes! In fact, I think I'll write up a 48-page contract, and you can tell any new girlfriend she has to sign it before she's allowed to date you.
Then it occurred to me that several people on one of the poly mailing lists I subscribe to are essentially doing just that, and they can't figure out why they can't seem to find any new partners.