I have shared this anecdote in other posts, so sorry for the repeat, but it seems relevant here as well.
I was 10 or 11, and home sick from school. Mom wanted to take my temperature before calling the doctor, but couldn't find (or maybe she had broken) the glass oral thermometer. As a result, she seemed genuinely sorry, but had to get a rectal reading.
I put up, not so much a fight as a bit of passive resistance, as one would expect from a pre-teen boy. However, I was secretly thrilled, since I had wanted to have it taken the "baby way" ever since having been switched (involuntarily) to oral several years before.
My heart was pounding as Mom came into my bedroom with the thermometer already shaken down and the end coated with a thick glob of Vaseline. She had me roll over onto my stomach and pulled my pyjama pants and undershorts down to my knees. After spreading my cheeks, she deftly inserted the thermometer. I still remember feeling the cold tip as it went deep inside. She then pulled up the bed sheets to give me privacy and left the room.
To my great embarrassment, Mom mentioned the event to my little sister, who came into my room with a little smirk and waited with me as the thermometer registered. She couldn't see anything, but knew very well what was going on, which was also somehow a thrill for me. It turned out I did have a fever, so was confined to bed for the rest of the day.
Unfortunately, my adventure came to a quick end, since a few hours later Mom bought a new oral thermometer, which she proceeded to use for follow-up measurements. To this day, I regret not having shown more receptivity to the idea of a rectal temp, since it just might have encouraged her to continue that way, at least for a few years. But the incident left a lasting impression on me, and still serves as a nice source of fantasy.