This was hot!
I used to live in an apartment building that was shaped like a 'plus' sign. My former neighbors moved out and rented the apartment to another young couple. The guy was kind of dorky, but his wife was a very hot blonde! They weren't really friendly but would say hello to my wife and I when we passed in the hallway.
One night I was watching tv when I heard some fire engines outside my building. I walked over to the window and pulled the shade to the side to look out. To my surprise, my neighbors were on their bed, next to the window, naked looking out too. I stopped and watched for a minute and then they turned the light off. I was kind of surprised to see that they had never installed any window treatments at all.
After that I began to check things out. Each night she would shower and then walk around her bedroom totally naked. She couldn't be seen by anyone on the street but the neighbors could certainly see her. I guess she didn't care. I would be hard in a second. Usually she would stand with her back to the window, looking in a mirror on the wall and brushing her hair. Usually she put on pyjamas, but not always. One night I saw her laying naked on the bed. Her husband walked over to her and stood beside the bed with an erection. She smacked it. I guess she wasn't interested. A short time later he reappeared and she used her hand on him. He left the room and then I watched as she spread her legs and began to masturabte.
I had my rock hard cock out in a flash and began to stroke myself while I watched her. My heart was pounding and I couldn't believe what I was watching. I don't think we were more than 20 feet from each other as I watched her finger herself and rub her clit. She had a beautiful dark blonde bush and very nice firm breasts. I had never seen a woman masturbating and I was mesmerized! When she finally came, her breasts were bouncing up and down as she threw her head back and her back slumped against the headboard. I absolutely exploded, my knees buckling as I peeked through the edge of the window shade.
I saw her naked a few times after that and then they moved. The next tenant moved in and put up shades the next day.