I don't know if they knew. The subject was never discussed
My mother was somewhat overprotective, so she used to take my rectal temperature every time she thought I looked sick or tired, maybe twice a month. I enjoyed those sessions because she was always nice and gentle, caressing my buttocks as the thermometer registered , and I must admit that from an early age I kind of liked the feeling of my ass being penetrated.
A small incident took place when I was twelve and my mother stopped taking my temperature. I had developed some kind of cold, and I discovered my mother had lied to me about the thermometer readings, telling me my temperature was 38.0 and informing the doctor one hour later it was 38.5. I was very angry at her and eventually made her cry.
So, later in this day, both my parents came to my bedsite and told me they had concluded I was too old to have my temp taken as a baby and that I could take care of measuring my re myself. And they gave me a brand new thermometer which I would keep in my bedside table drawer.
So, from this day, I had access to the thermometer and I took full advantage of this possibility.
At least once a week, I would take my temperature to enjoy the penetration of the thermometer in my rectum.
And a few months later, I started to masturbate and I quicky found out that it felt so much better with a thermometer in the ass.
This situation lasted a very long time, as a matter of fact, till I left my parents'house in my twenties.
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