When I was in my late 20s, I visited a urologist to have an STD exam (spoiler alert: it was negative). Part of the exam included a screen for genital warts (HPV) that required the doctor to apply acetic acid (essentially, vinegar) to the skin and head of my cock and to exam every square milimeter of the skin, shaft, and head close-up using a mono-magnifying lense. With a female nurse present (I had already changed from my clothes into only an exam gown, the untied in the front), the doctor opened the gown, applied the acetic acid, and then, after a minute, began to pull and stretch and move my cock around. And around and around. Pulling it, stretching it, moving it, looking at it.
Over the course of the exam, the acetic acid made my cock feel a bit numb, at least the skin felt that way. And because I was a bit uncomfortable being handled in that way by a man (at the time), and quite embarassed about the entire ordeal and scene, I looked straight up at the ceiling or had my eyes closed. So I was not fully aware that my cock was slowly becoming engorged, and that I was becoming a bit aroused.
I found myself feeling a bit flushed as the doctor made small talk and the nurse simply followed his directions, occassionally providing him with additional acetic acid that he would apply, but I nearly jumped off of the cold table when he said, "Don't worry about the erection. It's perfectly normal."
"What!?" I silently screamed, as my body went completely rigid.
"The acetic acid can cause that."
That was when I realized that not only was my cock hard and aroused, but that it was as engorged, thick and sensitized as it had ever been. EVER. It was as if I was being masturbated by a beautiful, skillful, nude woman; only I was not. It was a forty-something, reasonably good looking man, with a better-looking nurse standing behind him.
I was embarassed. I could feel my face completely flushed, but I could also feel my cock was super hard. Especially when I pushed my hips up ever so slightly. I suddenly went from feeling humiliated and ashamed to feeling aroused and surprisingly close to coming. I suddenly wanted to come. I pushed a little harder up as he pulled and stretched my cock from side to side, forward and backward, under the studious eye of his nurse.