The thrill of getting caught...
I'm a happily married guy, 37 years old with no kids. My wife and I live in the same town as her sister and her parents, all of whom we see frequently. In fact, her sister is at our house so often my wife has given her a key. She's a great gal, very friendly and outgoing and we get along great. Julie is somewhat older than my wife and I (45) and has a great outlook on life; she's pretty carefree and just takes things in stride. She is twice divorced (also no kids) and has a very nice job in sales. We have occasionally kidded and flirted with each other, but there has never been anything overtly sexual. I have seen her in various stages of undress over the years (the "most naked" was once when I walked into our bedroom and my wife and her were in the midst of trying on clothes from a shopping spree and they were both in bra and panties. That fueled many wild masturbation fantasies for months!).
Once every few weeks I have the opportunity to work from home and on this particular Tuesday morning back in August I took a break at mid-morning and, after a shower I was feeling particularly horny. Even though I have an active sex life, I also masturbate regularly (much to my wife's chagrin...) and when I'm home alone and have a chance I normally take advantage of it. So I plopped onto the living room couch in just my boxers and began to stroke myself slowly. As I became more aroused (I honestly don't remember what I was fantasizing about this particular morning) I stripped off my boxers and lay back on the couch naked. My feet were planted firmly on the floor, my right hand/arm were propped behind my head and I was leisurely stroking my erection with my eyes closed when I heard. "Oh, god. You are SUCH a pig!"
It was Julie. Obviously she had let herself in and I hadn't heard her come through the front hallway to the living room. I panicked and scrambled to cover myself with a pillow from the couch, embarrassed beyond belief. Instead of turning and walking out, Julie stood her ground and laughed. She told me she couldn't believe that I was "whacking it" and said that maybe I wasn't getting enough from her sister. I was literally at a loss for words, sitting naked in front of my sister-in-law and just having been caught masturbating. She laughed again and walked further into the room and sat on the reclining chair across from me.
I was caught completely off guard when she told me to not let me interrupt her and that I should continue what I had started. I was slack-jawed still and she told me to relax and not be so uptight. She leaned forward with her elbows on her knees and told me she had never in her life seen a man masturbate and that she had often thought about it. That's all she said. She then sat back in the chair and fixed her eyes on the pillow I had covering my crotch. I may be dumb but I'm not stupid (as the saying goes) so I likewise sat back and removed the pillow, revealing my now half-hard penis. Julie looked at me and smiled and simply said, "Very nice."
Being naked there in front of her was incredibly erotic to me. I had often fantasized about exhibitionism and was now fulfilling it. I began to fondle my cock, and was fully hard in a minute. Julie simply didn't move, she only watched with a mischievous smile on her face. I began to look her over, and noticed she had on a simple blue knee-length summer skirt with a neckline that showed just a hint of cleavage. Julie is a bigger gal with short blonde hair, and her breasts are VERY nice. As I sat there masturbating I very overtly stared at her legs and breasts. She shifted in her seat and tried to subtly hike her skirt up a bit and spread her legs slightly. It wasn't quite as subtle as she intended, but it still did the trick, as I now was able to see straight up her dress to the crotch of her white panties. She saw me lock in there and she smiled, and then her left hand went briefly to her breast and she gave it a squeeze. When I noticed her nipples poking through the light material of her bra and dress I couldn't hold back. I sprayed 3 consecutive spurts of semen all over my chest, and a few more spasms created a "lava flow" out the head of my cock and down the shaft and my hand.
As I regained my senses Julie stood up and walked over to me. She leaned over and kissed me on the cheek and whispered, "Thanks hon, that was wonderful!" She then disappeared into the kitchen, and I got dressed and went back to work downstairs. My wife came home about an hour later and the rest of the day proceeded normally. Nothing was or has been said by Julie or me about the incident and it will be etched in my mind forever.