My Theory: Status Humiliation
I was reflecting on what it was about rectal temps that effected me so much and more powerfully than other medical fetish procedures. The psychology is complex, but I believe the following is part of my fascination with them.
Like most people, it definitely relates back to childhood. I grew up at a time when RTs for children were common. You had very little privacy. If you were getting you temp taken at the doctor, it would be rectal and your pants and underpants would be all the way past your knees. It was an extremely vulnerable and submissive position. You had no choice and the temp being taken rectally was a sign of status.
I describe these as "Status Humiliations" They were things that were embarrassing but more so because they were being done solely because of your status.
Your status was that of child, you had no rights, no expectation of privacy. You were a hairless little boy with a tiny pee-pee and nothing to hide from the adults of your world who dictated to you. You were helpless and completely dependent on others for your very existence. These were very powerful emotions for a child and for me, the rectal temperature was the pinnacle of "status humiliation"
Surely there are other forms of status humiliation, like early bedtimes, being bathed by another, being undressed in front of others, like when I was very young and was forced to change out of my wet bathing suit right there on the beach. Something that you would never see an adult or even an older child being expected to do. You were being embarrassed solely because of your status.
Now combine that concept with the invasive nature of the procedure and the vulnerable position it puts you in combined with the stimulation of an erotic zone, I can see how rectal temperatures can become extremely powerful fetishes for people, myself included.
If this seems interesting to the group I can share some very powerful "Status Humiliation" stories to further demonstrate my concept. Perhaps I can write a PhD thesis on this? I would love to see everyone blush when I defend my thesis.