I don't think I was capable of verbalizing it at the time, but the contrast between the mother who bared my bottom so gently and solicitously for an RT, cupping my bottom cheeks with one hand and rubbing my back with the other, and the mother who yanked my underwear down and unceremoniously plopped me on her lap for a swift but painful bare-bottom spanking, was disorienting. I think what I learned above all (at first unconsciously) was that the child's bottom was the site par excellence where parental (and, in some cases, medical) authority was performed and expressed, sometimes benevolently, sometimes harshly.
In childhood, I did not have a temperature measured in the rectum (only in infancy), so maybe I should not comment. On the other hand, I had some experience, because I was spanked, I also had enemas and suppositories. Showing a child's buttocks is a symbolic demonstration of the parent's authority. I remember that when I was an older child, my mother expected me to lower my panties myself. It was an obedience and cooperation.
Mum immediately compared medical procedures (enema, suppository, injection) and spanking - an educational procedure. She said that one thing requires showing my bare buttocks. She said these treatments were unpleasant but needed. Thanks to injections and enemas I am healthy, thanks to spanking I am good.
Buttock procedures are also a mystery between the child and parent. I remember agreeing to the fact that I had to show my mother's buttocks, but I didn't want anyone else to know about it.
Plus, it only works one way - the parent can see the baby's bare buttocks, but the baby cannot see the parent's buttocks. This was the case in my home and in many of my peers' homes. It was an underlining of the parent's authority.