Thanks @Svensson for raising this topic. Simple question indeed. But I suspect that for most of us the response is anything but. 😊 For me, it is really a combination of intimacy, embarrassment and physical sensations.
First, intimacy. I love the idea that a caring woman partner is in control. “You don’t look so hot”, followed by a hand on my forehead and a worried look. “I’d better get the thermometer.” She would really like to treat me like an adult and take it orally, but I am unable to keep it under my tongue long enough for an accurate reading. So it is, “Sorry, but I’m going to have to get the baby thermometer and take it in your bottom. Take off your underpants and lie down on your tummy. I’ll be right back.”
Second, comes the embarrassment. It’s not just having to bare the most private part of my body. It’s the fact that she can make me do it. And maybe she’ll mention what is happening to another woman who is staying with us (Her sister or niece? The nanny? A friend? ...). “Have you seen the Vaseline? I need to use the rectal thermometer on him.” So there are discreet glances, maybe through the bedroom door, which she accidentally leaves ajar during the procedure. It is all the more humiliating if she makes me lie across her lap, as suggested in her child care book.
Later, at the the doctor’s office, she mentions it at reception so the other patients in the waiting room can overhear. “Yes, I needed to get a rectal reading because he couldn’t hold the thermometer in his mouth.” She accompanies me back to the exam room, even though I’m really old enough to go on my own. As I strip naked and get on the exam table in preparation for vital signs, she exchanges knowing smirks with the nurse, who announces, “Since you got a baseline rectal reading at home, I’d better do the same for a valid comparison.” As I turn beet red, she adds, “Don’t worry, it’s not just for babies. And anyway, it’s much more accurate this way. Now, turn over onto your tummy for me.” The two women chat while waiting for the thermometer to register.
Finally, there’s the actual feeling of penetration. The thick glob of Vaseline used to lubricate the thermometer accentuates the sensation of cold as the rod is firmly pushed into place, far enough in that the tip is touching the wall of my rectum. The cold lasts for about a minute, until the Vaseline melts and the glass heats up. As she holds the stem of the thermometer in place, the slightest movement on my part creates additional pressure that sends pleasurable impulses throughout my body. That in turn causes some reaction that, of course, accentuates the already deep embarrassment of the situation, whether at home or at the doctor’s office.
Of course, these pleasures of intimacy, embarrassment and physical penetration are compounded if I know that my partner and the others are also enjoying the situation, even though it is nominally a medical procedure. Catching sight of a flushed face and perhaps some discreetly erect nipples is enough to send me over the top.