Unexpected (but welcomed) Arousal

Where to start? Whelp, after my wife quit smoking she insisted that I no longer smoked in the house. So I would just go out into the driveway out the side door. I had neighbors just behind me and over one house. For years I could see the light from thier bathroom lit as they got ready each morning. I was always leaving for work so it was just routine. I should mention that their shower is on the second floor. And has a full sized window right near the shower head. They used the windowsill to hold their shampoo bottles and such. And now that I was smoking out side I soon noticed they had a nightly routine. As winter approached and the nights grew longer I could see them moving in and out of the room. The light going on and off as they went about their business.  One day I noticed a distinct difference.  The lighting was brighter and the silhouettes much more distinct. They had changed out the opaque curtain for a translucent one. I was surprised by this. Now I could see the perfect shadows of their bodies while they showered.  I would later find out they were selling the house. And must have changed it to let more light in for the buyers. A month or so went by and the nights got colder. At this point when I wanted a cigarette I would just poke my head out the door and use the storm door as a wind block. This left me looking in the same direction of their shower. About every other night I would catch him showering.  One night I stuck my head out the door and he was already showering. I have never met him but could tell he was mid to late 40's. Buy the way he moved and his belly. As I said, it had become normal to see this so I was just looking up now and then with nothing better to do while smoking. He turned to face the shower to do his final rinse. It was then I saw for the first time the clear outline of his dick. I had not seen it before. It was at the 45 degree chubby stage. Hanging visibly balanced between his thighs and stomach.  He the turned his back to the water. Giving an even clearer view of his dick just hanging out there. Now as a straight guy this is not a full turn on for me. But as an enthusiast of erotica I was transfixed.  He was doing something with his arms but I took no notice since I was focused on his obivus arousal.  Then it happened. He reached down putting his right hand around himself and started to go to town. It was maybe one or two starter stokes but then full middle speed pumping. I stood there in awe. Not sure what to think as my mind was taking it all in. And at the same time that familiar inner ache started at the base of my shaft. I sat there for a minute or two more. Watching as he stroked his now full erection.  The night was cold and my cigarette was done. Closing the door I went inside as he was still going at it. I sat on the couch for a minute with the intention of catching up on the movie we were watching. But as you guessed my mind was still on what I had just witnessed.  I started to explain to my wife what I just saw. She was at first shocked but understanding. She knows I masturbate daily.  A few minutes goes buy and I bring it up again. She pauses the movie to hear me tell the whole story a second time.  We started the movie back up, but about five minutes later I realized I was consumed by the show I had just seen. I looked to my wife and told her I needed to cum. She gave me that look of understanding and told me to do what ever I needed to do.  I stood up and stripped everything below my waist. Sat back on the couch, slid my hips to the edge of the and put my feet on the ottoman. Slid my fingers around my now 3/4 hard on and slowly worked myself. My wife asked why it turned me on so much. I think she was concerned about my sexual preference. I told her that it wasn't about watching a man masturbate. It was about the act itself. I'm sure there is not a guy out there who wouldn't understand. That moment of feeling while in the shower. Soaping up and getting aroused. The warm water, the slick feeling as your soapy fingers pass over the shaft. As you reach the end you swrill around the head. Then that moment, you know you want to cum. I had just watched someone go though that. And it got me wanting to do the same. That's how I explained it to her. As I kept the momentum.  Now fully erect and fully aroused. I put a little spit into my palm and let the feelling take over. The visual of him standing there in full view slowly slipped away as my own moment of pleasure started to take over. My eyes closed and I had one of the warmest orgasm I had had in a long time.