It was when I was 19 or 20, I believe. Think I was wearing just a gown with boxer shorts. I remember after the family doctor did the hernia check, he asked me to turn around and bend over the exam table. I wasn't expecting this, but didn't really think much of it, at the time.
He slid his finger inside, felt around a good bit, then was finished, and told me I could pull my boxers back up. It only last a matter of probably less than 20 seconds, but it sure left an impression on me. My head was still buzzing as I walked out to the parking lot. When I was driving home, as I was passing my old school I pulled in and went in to use the handicap bathroom, which was private. I was so aroused I couldn't think straight, and I was almost shaking all over. Also, the lube used made me need to evacuate. I took down my pants and boxers and was already half-erect. That bowel movement was much more of an experience than usual. My prostate had been aroused, I was now fully erect, and it didn't take much stroking before I exploded so powerfully that I needed to reach out to support myself on the wall. Nearly blacked-out!
A couple hours later, I did it again at home, although not so powerfully, but my mind was still all abuzz and I needed more release.
I'd played around with doing anal things to myself since probably 12 or so, but having this done to me by someone else was quite something else.