This happened when I was 14
I had never had a sports physical until I was in eighth grade trying out for the soccer team. There were rumors floating around that the Dr 'felt you up' It was rather stressful to think about at that age.
So the day came and they brought about six boys in at a time and we stood in one room with our underwear on. The nurse would do the basic and then one by one we would go into a private room for our physical. Finally somebody I knew well came out and I asked what happened and he said you have to 'drop your shorts'
I eventually went in and I was very nervous. The Dr was an older gentlemen who had probably done 1000's of these before. It was very routine and clinical but when the Dr asked me to lay down and he began to touch my stomach I started to get really hard. Hey..when you are 14 it doesn't take much. Of course right after that he asked me to stand and drop my shorts. At this point I have a full hard on but I had no choice. He did his thing with my balls and a quick check of my penis and I was done. It was embarrassing.
After school that day I was at a good friends house who hadn't had his yet. He had heard all about it but wanted to hear the details from me. I told him abut the drop your pants part and how he touched you etc. He wanted more details so he asked to show him. we had been friends for years and had seen each other naked many times so it wasn't a big deal. I had also noticed that he was pretty hard every time we changed together but I never said anything.
After a little protest I pulled down my pants and showed him on myself what he did very quickly. He then pulled down his pants and said something like 'he touches you here' So I demonstrated how the physical went 'to the best of my ability' on him and he got hard while I was doing it. He said 'I hope that doesn't happen tomorrow. I confessed as to what had happened that day and he seemed excited and intriqued. He asked if he could do the physical on me to see if he had it right. I let him do it and I was hard by the time I pulled down my pants.
We did some other touching that day but it was pretty conservative. He told me later the next day that he was really hard but it was a young male Dr so it was even more embarrassing!
This experience was the first of many others with him. I am now married but even at 40 I still worry about getting hard at the Dr's office!!