Interesting question, of which I don't have a clear answer. I like to picture in my mind almost everybody I see and know at the doctor's office. Male and female.
I don't fantasize about other men being examined, but I do enjoy picturing a male exposing himself for a genital exam and then getting in an embarrassing position (usually bent over the table) to receive a rectal exam. I imagine how I feel in those situations and how that compares to the male I'm picturing and what is going through his mind at the time. Is he embarrassed, does he enjoy it at all, does he shrink and shrivel or get hard, etc. The other thing I enjoy about picturing men receiving exams, is their relinquishing control to the doctor and being submissive themselves. I especially enjoy this aspect when I picture other doctors or people with bossy personalities or careers receiving their exam.
Everything I wrote about men, also applies when I imagine women receiving an exam. In my opinion, female exams are more embarrassing than males. I think it's because they also get breast exams, but mostly because they are face to face during the pelvic and rectal exam. All of their goodies are front and center and on complete display the entire time. Theirs also takes a bit longer and more steps involved. This is all generalized, I know it's not every person all the time. I wonder what it would feel like being a woman going through the exam and processes. All the thoughts and emotions and how I might feel about it. I'm really embarrassed during my exam, I imagine my embarrassment would be even more elevated during a female exam.
With women, I also like to imagine the entire exam from the doctor's perspective. How does she look naked, do her nipples get erect, is she wet, does she squirm, do her toes curl during the rectal exam, etc.
However, the biggest difference between me imagining a man and woman being examined is the sexual component. I'm not trying to start another debate of whether or not doctors or nurses have any sexual thoughts or urges during exams. This is just my opinions and thoughts. To me, anytime I am naked or exposed in the presence of a female, or a female is exposed in my presence, there is a sexual component or feeling. My mind is just trained to go to sexual thoughts, urges, feelings, desires, something. At least some small part of my mind. So when I picture women I know receiving an exam, at some point I picture a sexual side of it. So I almost always imagine having a sexual encounter with them also.