"Poor little sweetie, I'm so sorry you're not feeling well. Here, let me take your temperature."
"Wait what? No, not like that!"
"It's the most accurate way. Now be a good little one and stop fussing. This will be over in a minute or two."
"Hmmm... it says 98.6. I think you need a lesson in not making stories up as an excuse not to get out of bed. If the insides of your bottom aren't overly hot, your bottom itself is about to be. Lift up. That's it. Over my lap."
Taking a temperature rectally prior to a spanking is almost an archetype in spanking fiction, roleplay, and movies. I can understand the appeal. It enhances both the feelings of embarrassment, vulnerability and violation. Maybe it feels good. True confession- I've never experienced a rectal temperature as part of spanking play, though I have had a finger massage my prostate. Though I can't imagine a small thermometer providing the same unique sensation, I'll give them the benefit of the doubt.
However, I found it interesting that on fetlife, the fetish "rectal temperature" has a grand total of 373 people into it, while "spanking" has ~287,000 folks into it and just the jargonish "otk spanking" has 33,747 folks into it.
One might not guess that from just watching spanking videos, though. Rectal temperature taking is seemingly everywhere in spanking videos:
Maybe it's just the toothpick