So, thus far all we've heard about is couples branching out into monogamish arrangements and have to yet to hear from the one of the branches.
I am, more or less, the "piece on the side" to a married man in such a relationship. He and his wife met in college, he was only her second sexual partner, and after college and a few years into marriage he realized that eventually she would probably get curious as to what's out there and they opened up the relationship. They are in their mid-30s now.
I met him at work a year ago and we became pretty good friends. I'm 23 and I'm terrible at the whole dating thing. Not such a bad thing, I figure, because I'm young and I've got time to improve. And in the meantime, Dan, I wanna get laid like everybody else. I've met his wife, she's lovely and hilarious and knows exactly what's going on between us and personally gave me her blessing to sleep with her husband. I see him other every day but we probably only sleep together once or twice a month.
No one at work knows about "us." He's very discreet and worries for my reputation if people were to find out. I've told a couple close friends but they've never met him. All and all I'm quite happy with our arrangement and to the best of my knowledge, he and his wife are pleased with it too.
Pleased To Be One Of The Branches