There are a progression of thoughts when a rectal temperature is occurring. During my childhood and teenage years, the taking of a rectal temperature was mandatory at the doctor's. At home, mom was less rigid by the time I turned 10. So going for a physical was always associated with anxious thoughts and a growing awareness that I would have to undress fully or suffer having my briefs lowered, especially for a temp to be taken by the nurse while mom looked on.
I would first wish being told to strip wasn't being directed at me. Stalling only made it worse. It was a small victory when I kept my briefs on at least temporarily.
Seeing the nurse reach for the Rectal Thermometer and lube stirred my anxiety and I would feel myself tighten up all over my body. I tried no to look as if this meant she couldn't see me!
Being told to turn over onto my tummy meant it's happening. Even tears and mild protestations did not deter the nurse from pulling down my briefs to expose my rear end. Then her fingers spreading my tight cheeks meant, you are not going to escape. I would hang my head and shiver as I anticipated the insertion.
Her gentle lube of my anus did not make me feel good. The slow and steady insertion of the thermometer made me gasp and wish it was ending faster.
Lying there with it inserted and visible to mom was humiliation to the max. Hearing her chat as if it was nothing only made me upset, mad, and self conscious.
Upon it's removal there was relief but turning over meant exposing my penis and as I turned 11ish or certainly age 12, I was having an erection as a result of the thermometer in my rectum. I felt strange stirrings as my penis got erect and soon realized that ejaculation was likely.
It has all come, pardon the pun, full circle. Undress + expose my bottom completely +Rectal thermometer + insertion while on belly = massive ejaculation.