No, you're not the only one, SIL! Have you ever considered letting your mother-in-law see you or catch you masturbating 'by accident?' You might be surprised by her reaction.
When we were visiting my mother-in-law, I was in the bedroom. My wife had already gotten up earlier and gone out to do some shopping. I had an erection And decided to take care of it before getting dressed. I got out of bed and stood in front of the dresser mirror, because I enjoy watching myself.
It did take me long to get to the point where I was about to come, because basically all I wanted to do was relieve myself. But at the moment my mother-in-law walked in, bringing some fresh towels for us. I saw her in the mirror, saw the look of shock on her face, and turned around to face her, cock in hand.
She was red and flustered, as you might expect. But she didn't turn and leave the room, perhaps because she was just so startled and embarrassed. I was so close to coming that I didn't want to stop. Besides, I suddenly had the feeling that if I showed any embarrassment or tried apologizing, I'd lose the advantage (so to speak).
When I kept stroking myself, she managed to say, 'Stop that! It's disgusting!' But she couldn't take her eyes off me, either. In fact she even came a couple of steps closer, with this disgusted but fascinated look on her face. She kept saying, 'Stop that! How can you do that?' But she didn't miss a stroke.
I wanted to warn her and said, 'I'm gonna come!' Before she could react, though, I spurted with an intense orgasm all over the towels she was still holding, as well as hitting her blouse and slacks. She dropped her towels with a final yelp of disgust, while I stood there pumping out the last drops.
For a moment my mother-in-law just stood there. Then she said, 'How can you do that when you're married?' So that was her concern! I immediately said, 'All men do it, it has nothing to do with your daughter.'
She said, 'Well, it's still disgusting.'
I said, 'Then why didn't you turn around and leave the room? I didn't force you to watch.' She got flustered at that, and I suddenly added, 'Maybe you just enjoyed it and don't want to admit it.'
She just said again, 'You're disgusting.'
For some reason, all this talk was getting me aroused again. Normally it takes me a while longer to get hard after coming a first time, but this was obviously a special case. In another moment I was fairly erect again. I started stroking myself a second time and said, 'You can leave if you want to.'
I wasn't too surprised that she simply stood there, still with that disgusted but fascinated look on her face, while I slowly worked myself up to another orgasms. I took my time, wanting it to last as long as possible. And she watched avidly until I came a second time, nowhere near as strongly as the first time, but still enough to hit her blouse again.
At that she turned and walked out of the room, presumably to change her clothes. I called after her, 'Let me know when you want to see more!' But that's another story.