At younger ages, rectal temps were just part of doctor visits, and I was way more concerned with the inevitable shot in the bottom than the thermometer.
At around 8 or so I had an awareness that adults and even some other kids got their temps taken in their mouth while mine was still taken in my bottom. I did not like getting it taken “the baby way”. Shortly thereafter, I started getting erections from the procedure. I was pretty naive and didn’t fully understand it. I pulled my undies back up and I didn’t fit... I was poking out the waistband.
The older I got, the more I hated and resented the thermometer going in my bottom, but I also continued to get erections. At some point I figured out I needed to put “it” pointing up under my tummy to avoid discomfort. The moment would come where the nurse would tell me it was time for my temperature, and I would get very embarrassed as I got ready. The nurse would spread my buns and slide the cold greasy thermometer in, and it felt kind of good, and I would start to grow, but I was also keenly aware my mom and the nurse were watching everything, and sometimes the door would not be completely closed. When it was over I would try to get things pulled back up without showing my reaction, but if I was just in my briefs that was impossible.
My last rectal temp was by far the most humiliating of my life, I’ve written about it before, and I even successfully repressed the memory of it for several years. That happened at 12, and then I was switched to oral. It was a big day to finally be treated like a “big boy” even though shots would continue to only go in my bottom until 19. But like others, I would continue to play with the rectal thermometer at home and pretend I was going through the whole embarrassing ordeal with a very stern nurse, and these play sessions got me very erect and excited, even though the experience itself was humiliating and I hated it enough to try to forget it ever happened.