My wife was not amused when she found out, even several years later, but it still makes me horny.
My soon to be mother-in-law was called out of state to care for her sister-in-law who was badly burned in a grease fire. I was asked to temporarily move in to help my fiancée and her father take care of the home and gladly agreed. Also staying there and sharing a room with my fiancée was one of her girl friends, a year younger with a body to die for.
Some time later my fiancée was taken to the hospital very sick with appendicitis. It was so bad that she ended up staying in the hospital for two weeks. So that left only her friend and my future father-in-law to take care of the home.
One night as I lay on my bed I remembered my fiancée telling me that her friend J. slept in the nude. I figured that she would go to the bathroom before retiring, and I might be able to catch a look at her naked body. The parent's bedroom and bath were on the ground floor, my bedroom, J's bedroom and a second bath were on the second, so I wasn't worried about being caught spying on J. Sure enough, I heard footsteps in the other bedroom which sounded like they were headed toward the door. I figured that if I 'accidentally' ran into her in the hall she wouldn't have anything to complain about. I sneaked out my door and could see that the door to the bathroom was closed, confirming that J was inside. I just stood there by my doorway, completely naked, ready to intercept her as she exited the bathroom. As soon as I heard her hand on the door latch I started for the bathroom so that when she came out it would appear that I was headed there myself and that this was only a coincidental meeting in the hallway.
Sure enough, I was about six feet from the door when she came out and looked right down the hall directly at me. My heart was pounding in my chest, but J didn't look upset at all and I could see her eyes dart down to my crotch and then quickly back up. Now all I needed was an excuse to spend more time in her naked presence, so I told her that I missed my fiancée and asked her if she minded talking for a few minutes. She said she didn't mind and instead of going into her room to get something to put on she just walked down the hall and into my room still completely naked. She was 5'2', about 105 pounds, nearly waist length brown hair, small firm breasts and a tight smooth abdomen leading down to her dark brown curly hair covered mound.
I lay back down on the bed while she stood next to it and made chit chat, about what I don't recall. It was all a pretense, anyway, so what we discussed wasn't sticking in my head. We had to talk quietly as my fiancée's father was sleeping in the bedroom just downstairs and he would not be amused at all if he heard us. I had an awful time tying to keep from getting hard and it was obvious that she was looking. So I just flat out asked her if she liked what she was looking at, and she said she did. I made a comment about being horny, and sorry that my cock was starting to get hard despite my best efforts. She said that was okay and not to worry about it. After some more meaningless chatter my cock was standing at full attention. I'm only 5'8', 145 pounds but I have a nearly seven inch circumcised cock that is almost six inches round at the fattest point. I started to stroke it a little and touch my balls as if absent mindedly whilst still talking to her. It didn't seem to bother her, in fact she seemed quite entranced by it all.
Being really horny, especially with that hot naked body standing within arm's reach I asked her if she had ever watched a guy jerk off. She said 'no', so I asked her if she would like to watch me jerk off and she said she would. So I lay there watching her as I stroked and fondled myself for her. I really wanted to have sex with her right then, but knew I could not do that to my fiancée. All the time I was stroking myself and playing with my balls her eyes never left my crotch. Using some spit for lube I worked myself right to the edge of an intense orgasm. I thought I could stop and make it last longer, but I was wrong. My cock started to throb and cum spurted out all over my legs, chest, bed, floor, everywhere. I came so hard that I could hardly breathe.
As I lay there catching my breath she told me how awesome it was and she appreciated me jerking off for her. In fact, she liked it so much she had me do it for her twice more.