Ok ~ so I have this 11.5" thermometer, and you can check it out at my profile; there is a photograph of it there.
Candidly, there is nothing to taking a urethral temperature reading. And, yes, if you do it properly, the thermometer will slide down to your bladder opening.
There are some nice sensations, especially if erect, coupled with the mental aspect of ~ what happens if something goes wrong?
I've been a play.doc for a long time, and have had only one more-or-less minor problem arise. Fortuitously, I knew of a kink friendly physician in California, called him up, explained the issue, and he helped me through the issue.
And, on that subject, if you do encounter a problem, don't be shy about going to the ER, a physician, or a doc-in-a-box. The ER has seen everything imaginable, so you might be a little em bare assed, but that's better than having a major medical problem.
Now, go out and make it a gr8 weekend!