As a kid who was in the hospital at least a half-dozen times during the late 50s and first half of the 60s, I was often subjected to rectal temps sometimes daily. And when I was given enemas at home, my rectal temp was taken first before the nozzle was inserted. This was routine practice until I was 13.
When I was 12, I was in the hospital to have my tonsils taken out. The morning of the tonsilectomy the nurse came in rolled me on to my side and then asked me if I wanted to put the thermometer in my bottom or wanted her to do it. I said I would and she handed me the thermometer with some kind of lubricant on the tip. I inserted it in myself but the nurse took it out. From that point on, I started enjoying taking my temperature with the rectal thermometer and that enjoyment still exist today.
When my uncle took over giving me my "usual" enemas around age 10, he would often take my rectal temp first. He said something about detecting an infection in my bowels and "playing it safe". If my temp was high (fever), he would not give me the enemas. My aunt, a nurses aide, would check me out by taking my temp orally to see if I had the flu or something. I remember a brief conversation (or maybe an argument??) between my aunt and uncle with regards to taking rectal temps. Don't recall if it was a general discussion about the topic or if they were talking about me.
When I take my enemas today, I ALWAYS take my rectal temp with an old-fashion rectal thermometer. It adds to the enjoyment of the enemas, for both health and pleasure. And when I enjoy enemas with a partner, usually a male, I insist on having my temp taken first and also take his before giving him any enemas.
So, for me, rectal thermometers, suppositories and enemas all seem synonymous with one another. Getting your temp taken at any age, over or under 21, seems to fall into a pattern of doing things the old-fashion way. I don't trust electronic devices because they're not always consistent and subject to faults; glass rectal thermometers, on the other hand, are always accurate. And, by the way, I don't use the mercury types anymore for safety reasons. I prefer the "newer" rectal thermometers with the red liquid indicator. Easier to read and not as dangerous as mercury if they get broken.