I sent my plan a few days ago: https://www.reddit.com/r/sexystories/comments/83dyz0/erotic_anniversary_m_f_str8_voy_swing/ …
… and it took place :-)
I played a erotic massage video to my wife. I explained to her that there’s no sex involved, “only” masturbation during the massage. I told her to choose if she wanted a man or a woman, and what she wanted for me.
A few hours later we were getting naked in the establishment. Nice room, temperature and smell, a very cozy place. We were nervous and laughing because of that.
We lied on our stomach and waited for a minute until the masseurs got into the room. We almost couldn’t see them from our position, but I could see them removing their towel. They weren’t fully naked. She was wearing black knickers and he was wearing black underpants.
They begun massaging our feet and legs. I was touching my wife elbow so she were calmed and felt me next to her. Their hands moved to our butts, just a little, and then our back and arms. I could see him crawling over the body of my wife. When my masseuse touched my balls between the legs for the first time I looked at them and saw his hands between her legs. That was astonishing. We smiled each other.