Yeah, for a brief time. We met through a gay online service in Seattle where we lived, and he told me he and his partner had an open relationship, in which he was allowed to fool around. We agreed to meet at his place for the first time, the partner at work, and soon we were kissing on the sofa and fondling dicks, then decided to head for the bedroom.
I could see the master bedroom at the end of a hallway, and for some reason I was walking ahead of him.
"No, not there," he said behind me. "In here," as he pointed to another doorway along the hallway. "That's our bedroom, I don't wanna do anything on our bed with somebody else."
That made me wonder if theirs really was an open relationship, or if this was a simple case of cheating. But my hardon was already sticking out past my open pants, a little late for me to be having second thoughts, so I followed him into the guest bedroom.
The sex was fantastic, BTW, and we got together a few more times at his place, once going onto his cabin cruiser that was docked there at his lakefront property on Lake Washington. But our schedule was built around things like when his partner was out of town, making me increasingly uncomfortable with the secretiveness, so I decided to break it off.
About 6 months after I stopped seeing him, a friend of mine who knew them both said they had split, each moving away to different states. And he confirmed my suspicions, saying my "trick" got caught by his partner with another guy, causing a very nasty break-up. The details of which flashed around the Seattle gay community, because they were both prominent & wealthy, the other partner additionally being a member of the gay Seattle Men's Chorus (SMC).
[I went on to have a very strange couple of tricks with the SMC's Business Manager at the time, which makes a funny story on its own]
I'm just glad it wasn't me who got caught with him. And I've never knowingly cheated with another married guy again.