As my girlfriend Deborah and I use a lot thermometers when making love, I would say it is usualy completely naked.
But, as I have mentioned on this site, l had an humiliating but exciting experience with RT as a young female school doctor took my rectal temperature at 18 at my last high school physical. I told my girlfriend about this event and one of our favorite sexgames has been to replay this scene.
Deborah has me stand in front of her sitting and, as the doctor did years ago, she lowers my briefs to my ankles and palpates my penis and testicules. After that, with a horny smile, she asks me to turn around. She then parts my buttchecks and, without any notice, she inserts deftly her thermometer in my rectum.
Then, as the thermometer registers, she may just keep her hands on my buttocks as I told her the doctor did, or more frequently she may play with the thermometer, making small in and out movements as if she was making love to my ass.
Once, as we were having this play and Deb was in a horny mood, she instructed me to masturbate as she was playing with the registering thermometer. After that, believe me, we had a wild sex session.