I have an exhibitionist streak
This happened when I was 16.
I had a sports injury to my knee and had to have ultrasound to try and help it, and I went to the physio a number of times. Although I can't now remember her name she was a good looking woman, albeit considerably older, being I think in her 40s. She was quite thin, with bobbed hair and quite attractive. However, it wasn't so much her as the physio which was the point.
The ultrasound used a small hand-held device, which to be honest I can't really understand how it was supposed to help. However, it did involve me having to strip down to my boxer shorts every week, whilst she applied KY jelly to the device and then rub it over my knee.
The device gave a gentle buzz, which coupled with my overactive teenage hormones and being almost naeked from the waist down, led me to having regular hard ons! Thankfully because I wore boxer shorts they made some effort at covering my embarrassment but as the weeks went by I started to get slightly less inhibited.
One day as she was luving up my knee I felt the familiar tingle in my groin. Being a young impressionable man I thought that things like this happened all the time and that she might grab my cock in some sort of 'Mrs Robinson' style sex romp! So, as my cock got harder and harder I didn't try and hide it. Eventually it started to poke right up out of the fly of my boxer shorts. I couldn't believe how turned on I was. Here I was, lying on a physio bed my hard young cock sticking up in the air right in front of a fully grown fit woman who couldn't have missed it!
In fairness to her professionalism she did try and ignore my hard cock, but it twitched every time the buzzing of the ultrasound ht my knee. I can remember my heart pounding in my chest as I was so embarrassed and yet turned on at the same time. By this time I was just starting to leak a tiny bit of pre-cum. I'm not a big 'pre-cummer' but the situation was really getting to me. She must have seen this because as my cock's eye opened and the sweet liquid seeped out of the top she stopped and told me really strictly to 'Put that away!'.
I couldnt believe it, she managed to hit just about every sexual button that I have! I have to confess to love being told off when it comes to sex/masturbation. There's nothing I like better than feeling that what I am doing is naughty. Plus I have always had a thing for older women, and here she was, dressed up in a white sort of doctors/nurses uniform, telling me off for having a hard cock!
My arm trembled as I went to put my throbbing cock away. It seemed to take so long as what she said sank in, the embarrassment, the scolding shame, and the absolute thrill of it all. Such that, by the time my hand found my stiff member I was almost climactic. All it took after 10 minutes of the buzzing vibration working up my leg and the visual stimulus of this sexy older woman was the slighest touch of my hand as I went to tuck my self back in and my orgasm hit me.
It felt electric, like the ultrasound amplified a million times, my whole body shook, my heart nearly exploded from my chest as I shot countless ropes of cum all over myself and my boxer shorts. The shame was intense but the feeling was incredible!
She was clearly disgusted with me, but I had no control over my body, and was still lying there, panting in shame as she handed me some tissues to clean up after myself.