This all happened about 15 years ago when my wife (Louise) and I moved to where we now live from out-of-state. We arranged to stay with her widowed mom (Pat, who was about 50 and a bit heavy but with huge boobs) while we looked for a home of our own.
Mom-in-law was very good at 'accidentally' walking in on me in the shower, or when I was getting dressed. She didn't knock on the door, just charged in. I once complained to my wife that one of these days she would walk in when we were having sex, or when I was jacking off. All my wife said was that her mom had been alone for several years, maybe I should just put on a show for her and get it over with. I didn't think she was serious.
At this time I was a night worker and slept during the day while my wife worked a regular day shift. My mom-in-law didn't work and was home during the day. She would usually wake me at about 4PM, a couple of hours before my wife got in from work, and we would watch TV and chat while I woke up.
Some days I would wake a little early and if I felt horny I'd have a quick jack-off to relieve the urge. One day about six weeks after we moved in I woke early and was laying on the bed enjoying myself and about to come, so I threw the sheet off, let out a groan and had a great orgasm, laying there with my head back, eyes closed, covering my chest and belly with sperm. As the feeling subsided I kept squeezing my twitching cock and when I opened my eyes I saw my mom-in-law standing in the bedroom doorway, arms folded across her chest and with a big smile on her face and her eyes fixed on my cock. All she said was 'Having fun?' and backed out of the room smiling, pulling the door closed behind her. I had no idea how long she had been watching, but either way she had certainly seen enough.
I cleaned myself up, pulled on a sweatsuit and sheepishly wandered into the living room where she was watching TV. I sat down and after a minute or two of silence I said to her 'Look, I'm really sorry about that, I just didn't hear you coming', to which she replied with a smile 'Well, I sure heard you. But look, don't worry about it, you wouldn't be a normal guy if you didn't do that from time to time'. I was stunned. While certainly not a prude, we had flirted in a quite obvious way and discussed her superb boobs before (much larger than my wifes) but she had never given me any reason to think she was that open minded. I asked her 'How long were you there? I mean, how much did you see?' and she told me, still smiling 'Oh, I saw it all, from the first groan until you spotted me. Look, it's been a long time since I saw anything like that. I'm a normal healthy woman. I don't want to make you feel strange but I enjoyed the show. Where's the harm in that?'.
The really amazing part of this episode is that when my wife got in from work the next day she asked me to come to the bedroom as she needed to talk to me. 'Mom told me last night that she got an eyeful of you 'amusing yourself'' she said (her euphamism for when I masturbate, which we both love her to watch). I admitted what happened and expected her to be angry, but instead she was fascinated, almost turned-on by what had happened. She asked lots of questions like 'How did you feel when you saw her there?', 'Were you turned on that she saw your cock and all your spunk?' and 'You know she saw the whole thing, from the first spurt until you were done?'. Then she dropped the bombshell with 'You know, she isn't stupid. She must have known what you were doing in here. That's why she opened the door so quietly. She wanted to catch you 'amusing yourself''. I had to admit this made sense. Then she went on 'You'd better watch yourself. She told me she enjoyed the show and if she gets half a chance she'll 'accidentally' catch you again'.
Although my wife never admitted it I had the feeling that she and mom-in-law had set this up. My wife had taken pictures and video of me jacking-off and I suspect that she had shown these to her mom, who then wanted a live show. She had shown the movies to some of her friends and this didn't bother me, no-one has to look if they don't want to. I think she liked it that her mom had 'accidentally' seen me naked and she liked that her mom had 'accidentally' seen me masturbate and ejaculate.
I have to admit that this episode was quite a turn-on when I thought about it afterwards and got over the embarrassment. Mom-in-law did 'accidentally' catch me on several occasions after this, eventually becoming quite brazen about it and just standing and watching until I came. My wifes only rules were that there was to be no physical contact between her mom and me (pity, I would have loved her to jerk me off and I think she would have enjoyed it too) and that I had to tell her if I was 'caught' and describe how it felt to have her mom watch me.
Mom-in-law also decided to give me some J.O. material, she would 'accidentally' walk out of her shower and in to the living room wearing only a towel around her waist when she knew I was around, then pretend to be surprised and cover her boobs with her hands (which I found incredibly sexy as she had large boobs and small hands) or she'd make sure that she came out of her swimsuit top when she pushed herself out of the pool. Sometimes my wife would be there when she did this, sometimes not. My wife liked me to jack-off while telling her about seeing her moms boobs or her mom watching me come.
Unfortunately mom-in-law passed away a couple of years after this episode, pancreatic cancer, so it never went any further than the occasional show, but it has made me a lifelong lover of jerking off for a female audience.