Great question.
Answer, A few things.
It taps into a fascinating, intense period of life when hormones and sexual feelings were beginning, early puberty, but when the only touching of those sexual areas was done in the privacy of a doctor's office. I didn't play sexually with a girl until my mid teens but I'd been naked at the doctor's office and had my private areas examined and touched for a few years leading up to that. Those were confusing, embarrassing, exciting experiences. It also sparked a lot of thoughts during school, wondering if the pretty girls in my classes had those same things done to their bodies at the doctors?
Medfet involves nudity and the examination and penetration of areas of the body that are very very sensitive and that can bring so much pleasure, if the "doctor" is not a real doctor and is actually a friend or girl/boyfriend or wife/husband. Medfet can be all the good parts of medical experiences without the negatives like illness or worry or expense or unpleasant doctors or nurses.
Medfet gives you the excuse to do naughty, nasty things to your sexual partner. Like a little wink, like "it's not that I want to do this to you, but you see I need to do this to you because it's medically necesary". I used to call my girlfriend at work and say I was calling from "Dr. Richard's office" and ask her if she would be home tonight for her full physical exam. If she said "yes, I will expect Dr. Richard at 6pm", then the scenario was set, I packed my doctor kit, and everything I told her to do from then on was a MEDICAL NECESSITY. Wink wink! I definitely took liberties with that woman's body and she never objected.
Medfet leads to some interesting discussions and memorable moments between two people. For me, quite a few discussions about my own and a few girlfriend's medical experiences and memories.
An older colleague and I began seeing one another and by the end of our first weekend together, I had got up the courage to explore her bottom quite intimately. Afterwards, our conversation led to her telling me about the first enema she ever got, when she was 12 years old and how her mom held her hand during it and that it was so embarrassing to endure but that she'd thought about it ever since and how it made her desire to have her bottom exposed and examined and penetrated again.
A woman I dated for about a year put on her old high school cheerleader uniform for me one day. It was a little tight but she looked so gorgeous and I jokingly asked her if she ever went to the doctor's office dressed like that. She said it happened once. I said "I bet the doctor was pretty excited to examine you!" She said she didn't think so, that he was very professional, and of course she was only wearing a gown for the exam, not the uniform. I asked if her mom stayed for the exam, she said Yes she did. I asked if he examined her privates, she said Yes, he did have her spread her legs and he gently parted her labia but only to look not penetrate, and that once turned over he spread her bottom apart for a quick look at her anus but did not touch her there. Of course I was very excited once this conversation was over and I asked her if I could play the doctor and examine her right then and there in her uniform. Fortunately she said yes, but I already knew she loved playing doctor. It was extra exciting to keep her in her cheerleader outfit for as long as possible, like to have her bend over, then flip up her skirt and pull down her panties to her knees as I examined her bottom. Of course the vaseline and thermometer were next.
My college girlfriend had a painful pilonidal cyst between her buttocks and went to a hospital for care. She called me afterwards and explained how the area between her cheeks was examined first by a nurse then by two different doctors, including her first digital rectal exam. She said she was happy to get the medications but was embarrassed that her bottom was examined so much in one visit. And that one of the male doctors examined her then didn't cover up her bottom for the rest of the time they talked, so she was laying facedown and panties down for quite some time as he explained how it's a common condition for truck drivers and on and on. And for the DRE the female doctor didn't get her finger in the first time, applied more lube, told her 'you have a very tight anus', and that it was quite painful. When I went over to her apartment later, she let me examine her for some time, noting the oozing cyst several inches above her anus and also getting a light to look at her anus up close and asking her again what it felt like to have the doctor's finger in it. Some weeks later I asked her if I could put my finger in her bottom but she didn't seem to be in a role play or play doctor mood, not really her thing. Eventually she did allow me to, and although I used a lot of vaseline, she didn't enjoy it and said it hurt, and I admit the doctor was right, she did have a very tight anus. But I must say it was an incredible view to see her with her head down on the bed and her round bottom up and spread and glistening in front of me and my finger pushing into her all the way to my last knuckle. It made me think "I can't believe people get to do this for a living".
My own memory of that first doctor visit as a pubescent 12 year old boy remains so intense, watching & feeling in near shock as the doctor examined my penis and testicles very thoroughly, then to my complete surprise and embarrassment to be told to turn over and suddenly feel his warm hands opening my buttocks wide for what seemed like forever, to examine my anus. This story I've only told to 3 women in my life, including my current girlfriend, all of whom listening with wide open eyes and grins on their faces. Needless to say at the hands of those 3 women over the years my bottom received many many visual and digital exams and rectal temperatures, including some exams that probably don't exist in the medical textbooks.