Several years ago, I had to go through 8 weeks of physical therapy to heal from a torn shoulder muscle that required surgery. The physical therapist was a very qualified but somewhat shy guy (Mike) who is several years younger than me.
Most of my therapy work involved 'passive' exercise. I had to use my good hand to guide and support the healing arm. In one particular exercise I would bend over with my feet open shoulder-wide and grab a stationary bar on the wall while letting my healing arm hang loose. I would then start making small circles with my legs and hips. This created a pendulum to strengthen my arm muscles.
I spent three hours a week with Mike, and over time we warmed up to each other. He would occasionally hold my arm as well to teach me correct and safe ways to exercise. One day into the sixth week he held my arm and by accident lightly touched my breast. His touch caught me by surprise, and my nipples became erect. I could not wear a bra and so my condition was embarrassingly obvious. There was sexual tension in the air but neither of us said anything. He continued moving my arm and ever so lightly touching my breast. I tried not to acknowledge what he was doing or how good his touch felt, but my breathing and his became faster. He then told me to move on to the pendulum exercise. I was praying it would not obvious how horny I was, but then I saw that Mike was horny as well, judging by the big bulge in his pants.
I started the pendulum and Mike remained a few feet away from me. I could almost feel the heat of his eyes over me. My heart was racing and I tried to concentrate on the exercise and the slow hip circles that it required. After a while, Mike got closer to me and almost casually pressed his groin against my undulating buns. I could not believe what was happening but kept on moving, making sure to caress his engorged penis with my gyrations. We ground each other's skins without any word or sound other than our short breathing. The more he pressed against me the more forcefully I pushed back. We were slow dancing, my panties were soaked and I only wanted to continue forever.
After perhaps 10 minutes that seemed like hours, Mike became still, held my thigh strongly and suddenly exploded in a powerful series of quakes separated only by faint, almost animal sounds. His orgasm caught me by surprise, and I came. My legs were weak, shaking without control, and I desperately braced myself to avoid falling on the ground. The muscles in my vagina were contracting like crazy in synchrony with the shocks that were running throughout my body. I let out a low and long whimper of pleasure.
By the time my orgasm subsided and I regained control of myself, Mike had left the room. Shortly thereafter it was time for me to pack things up. Mike and I never talked about what happened that day. We went back to our own respective roles as patient and health care provider. I have not seen him for years but sometimes I wake up in the middle of the night, horny and wet, and cannot help but touching myself thinking that I am still grinding for dear life myself against him.