When I was growing up (40 years ago), there were no electronic thermometers… or perhaps there were, but they may have been very costly and their use confined to examination rooms, I don't know, in any case the first one I saw, I was a teenager. Thermometers were glass. Earlier ones contained mercury, but at the time I was growing up I think these were being phased out due to the hazards when they break. I *think* my parents had one mercury thermometer and another thermometer or two containing a blue liquid. I also recall seeing a thermometer with a red liquid.
These thermometers looked like this
https://en.zity.biz/gallery/image/263997
or this
https://en.zity.biz/gallery/image/263996
They consisted of a thin tip, which could be inserted in a bodily orifice (mouth or anus) and a thicker part which would normally be left outside. Yes, I know this is different from the way thermometers typically were in North America. I think this kind of thermometer was commonplace throughout Europe. The same thermometer was sold for oral, rectal or axillary (armpit) use. There were no "rectal" or "oral" thermometers. (The only separate kind of thermometer that I heard about was the "basal" one, for high precision measurements of body temperature to determine ovulation. This one was to be used only rectally, due to the high precision needed.)
My mother considered only rectal to be reliable. I don't know if it came from personal experience of unreliability, or because she had read or heard that. Nevertheless, only the rectal way was used at home. As young children, a parent would insert the thermometer, and older children and adults would so some themselves while under sheets in bed. The thermometer would always be cleaned with alcohol after use.
This was I think quite a common situation back then in France for adults to take their own rectal temperature. On the one hand, I recall a girlfriend doing so in front of me. Furthermore, nurses hospitals back then in France expected older children and adult to do likewise - they would hand the thermometer and you had to insert it (I suppose that if you were unable to, they would do it for you). On the other hand, my wife suggested to me that her parents did not have her take her temperature rectally, and her reaction when I asked about when she went to hospital as a kid suggests she had a rude awakening there.
I read similar experiences on the French Zity forum, so I guess that what I witnessed was quite representative of that era.
When electronic thermometers came in, I think people continued using them in the same way as before. The first electronic thermometer my parents had had a rigid tip, but later came the flexible tips one, and the only reason you'd want a flexible tip, I think, is for easier rectal introduction.
Other kinds of electronic thermometers were also introduced, but many were unreliable. However, it seems that nowadays the infrared "guns" are quite reliable and provide a very quick method for checking temperatures without the embarrassment, need for privacy, disinfection needs of the rectal route. It's now become old fashioned to use the rectal route. I came lately by accident across a conversation on a social network in which a lady mentioned her young nephew's temperature being taken rectally and some people were reacting "poor child! nowadays there exist more modern ways!".
I saw a video lately in which a woman searches a bag she found and comes across a thermometer, an electronic one with a tip, has a laugh, and says "hey this is a BUTT THERMOMETER". Evidently for her, a thermometer with a tip, as opposed to an infrared gun, must go into the behind, not the mouth.
Today I think it should be quite common for people over 45 to take their temperature rectally, because that's how they were raised, but uncommon for people under 30.