Apologies for the couple of links in blue and underlined. They lead to advertisements and it seems as though I've got an Ad bug.
There are two issues here, about mercury filled glass thermometers. Another about glass thermometers per se, regardless of what they are filled with.
To have a glass thermometer break when inserted in the rectum, or, off Topic I know, the urethra, is tantamount to a disaster. There is almost certain to be injury necessitating a visit to the ER and probably further non accidental (cutting perhaps) injury when the pieces are being removed - as they must of course be!
If mercury is spilt then there is a further risk from the mercury. 'Flushing the system', which marcus master mentions, is not an option - any damage is done!
Inserting a normal glass thermometer in either orifice is risking breakage, especially if the fabled excitement at the procedure produces involuntary movement, particularly muscular spasms. Breakage is less likely in the rectum than the urethra because the only place of close contact with the body from rectal insertion is in the relatively short anal canal whereas in the urethra there is more constricted contact for a greater distance. That is of course assuming that that the patient/user does not move too much, perhaps while being further stimulated, or sit down with the thermometer inserted.
We read here of folks buying candy or industrial thermometers because of their greater diameter and thus the greater stimulation. In their intended use though they are almost invariably meant to be used mounted on a plastic or wooden board - that will clearly be removed for rectal use unless ones anus is a rectangular slot! All of this means that they will be thinner glass because they depend on such support.
If a properly designed and manufactured rectal thermometer is used then the risk is lessened, but not eliminated. I wonder how confident we can be that purchasing a rectal thermometer from an Adult store etc guarantees that it will be of adequate design and manufacture. Because rectal temperature taking is almost never a true medical procedure these days there cannot be much of a genuine market and thus not many genuine manufacturing facilities either. So China etc fills that gap, with not a great deal of concern for safety, I'd vouch.
Yes, I take rectal temperatures during consultations. I buy electronic ones in mini-bulk lots, probably a dozen at a time. I regard them as disposable and offer them as a souvenir to take home. OK, they penetrate but an inch or so and may not be entirely accurate. The accuracy I test as best I can by pushing the activation button through the blister pack and I discard any that are not within O.1C of each other before I go to pay for them. But is accuracy a real factor? And what is that persons 'norm' anyway? It's the experience of the procedure that most are happy with.
If you want to penetrate further - or use a thermometer in the urethra/vagina, then there are plastic ones filled with alcohol, usually yellow or red in colour, and I commend those to you. There are electronic ones too, incorporating a probe on a flexible lead - a proper medical instrument. Can you afford one?
Please folks, don't treat this subject with a cavalier attitude. A play examination is meant to be fun, not an ordeal or worse. Apologies for the couple of words in blue and underlined, which link to an advertisement. Looks like I've got a bug!