In France, when I was a kid, the most common way to take one's temperature was rectally, for children and adults alike. The only difference is that younger children had their temperature taken by some guardian, while older ones or adults did it themselves. It was not considered kinky or weird, it was normality (though in some families, they did it orally or under armpits).
So for a majority of people over 40. I'd say, rectal temperatures are just the way they were brought up. Perhaps it's different for people in their 20s.
Taking one's own rectal temperature today would be considered perhaps a bit old-fashioned but not kinky.
Taking one's spouse or partner's temperature could be considered kinky, or just a way to have fun with the unpleasantness of sickness.