I personally don't do the relationship thing, but I've banged a few open relationship chicks.
One story comes to mind. Years ago I was playing a gig, and a bunch of friends of the band showed up, including this pretty girl. I ended up talking with this girl through out the night, we hit it off. At the end of the night she asks "When are you playing again?", I said "Next weekend". That was it.
So, next weekend comes around, she shows up, but solo, no other friends. No biggie, she hangs with the group during breaks, we chat again, get along great. "When you playing again?"..."Uh...next weekend".
Next weekend comes around, she shows up solo again. When we finish our first set, I walk over to her table, where she's alone, I sit right beside her, very close...I decided, fuck it, gonna give her a kiss, all the signals are there. I lean over to give her a kiss, and I hear "Hello!"...it's some dude who joins us. He's smiling, she's smiling...I'm like "Wait...what?". "Oh, this is my husband"...he sticks out his arm to shake my hand, "Heard nice things about you, hows it going?".
Yeah. You see, she never wore a wedding ring, never mentioned being married, or even in a relationship. So, they go onto to explain they're sorry for blind siding me, but they have an open marriage, and their deal is they have to meet the other person, at least once...so...SURPRISE!
The three of us sit there, he orders a bunch of drinks/shots (oddly he knew my drink of choice, his wife must have told him). We do a bunch of shots, enjoy our drinks, and end up having a great chat...just shooting the shit. I remember thinking how relaxed both of them were, how sure they were. The break ended, he says "I'm off, enjoy my wife".
So, I did. I fucked her for a year, until he got a job on the other side of the country, and they had to move.
Coo people.