My most memorable moment didn't even involve sex. We were having a custom home built. The builder I'd selected was about our age and my wife thought he was good looking. His wife was a complete knockout. We became more than clients and had become pretty good friends during the months the house was being built.
One evening we were at their home. During conversation something was said about something and a certain man's name was mentioned. This was a man that my wife and I had swinging experiences with and we, instantly, knew that if they knew him too then we shared something special with them.
There was a moment of silence as all four of us knew what had been revealed, then there were big grins all around. That sure expanded the friendship.