First floor window
When my wife and I were living in our first apartment, she would leave for work earlier than I needed to so I would take my shower after she left before I went to work. One day as I was getting dressed I noticed that our upstairs neighbor who was friends with my wife would walk by our first floor window to get to her car. The next day I opened the blinds so that they were about a foot open. When she walked by the window, I walked by completely naked. I couldn't see her but I heard her stop. I walked back and actually stopped in front of the window. After a couple of minutes I heard her walk to her car.
I did this everyday for the rest of the week and on Saturday my wife told me that she had invited her friend and her husband over to watch the football game on Sunday. I was concerned that my wife's friend had told her husband about my exposing myself to her in the mornings. Nothing was said until halftime when my wife said that since her friend had seen me naked that she wanted to see her friend's husband. He quickly stood up and undressed and walked back and forth in front of my wife who was sitting on the couch. Then he took my wife's hand and they walked to the bedroom. His wife and I stood in the bedroom doorway while he undressed my wife and she laid back on the bed. He was fully erect and got on top of my wife and penetrated her. His wife and I watched while he fucked my wife and came inside her. Then his wife and I took our clothes off and she laid down beside them. I got on the bed and fucked his wife while he and my wife watched.
After all four of us laid there a while, we switched to our own wives and fucked them until we all came. We continued having sex together for almost two years until they moved away for him to take another job.
My wife surprised me after not talking about that period in our lives for years by bringing it up. She has recently been in touch with her friend and she had moved back to town with her new husband. We got together Saturday and the four of us spent the night and all day Sunday in bed. While I enjoyed having sex with her again, my wife got to have sex with another guy.