Difficult for me to answer this question.
At 73, I think I can say I have had a strong rectal temperature fetish since I was born.
When I was 13, my parents stopped taking my rectal temperature, they gave me a brand new glass thermometer witch I was free to use. So, from my teenage years till now, I have been taking my rectal temperature more or less regularly. And I have been masturbating a lot with a thermometer in my butt.
If we speak of a third person taking my rectal temperature, it gets a bit more complicated. Does having my rectal temperature taken standing up by the female schooldoctor, just as I turned 18 count.
Then, speaking of girlfriends, my first girlfriend, from 21 to 24, was strongly opposing rectal temperature as a principe. She never took my RT and never let me take hers. My second girlfriend, from 25 to 28'was just the opposite. I have posted about her on this site, Maria was very much into medfet and she had set the rule that she would give me a comprehensive medical exam everytime we were to have sex. This mock exam allways ended with her taking my rectal temperature on her lap. Of course, I enjoyed it a lot. .