Many years ago, I was having an 'affair' with a married woman.
I didn't know at the time, but her husband was aware and approved.
Chatting on the computer one night, she tells me her husband has an interest in watching us and joining in, and he began to join in our conversations.
I was teasing him one night, asking if he would like to see pics of my wife naked. After his wife left the convo, I showed him 2-3 pics, and he was pleased.
So he and I began to have more private convos, and I started showing him more pics each time. He would get a hard on while looking and commenting, which made me get hard, and we would watch each other jack off.
I still have a fantasy of doing the same or similar in person, but each year seems less and less likely.