When I ask for a play exam from a play doctor who identifies as straight, I will usually be turned down flat just because I am a male and not a female. If no sex is involved, then why should it matter who is given an exam? Does a real life doctor pick and choose his or her patients for exams? Certainly not! They take whoever is seeking an exam with them.
i think this is a good and interesting question. i wonder if it has to do with the individuality of our sexuality, and how layered and complex it can be?
No doubt, there are straight guys with a med fet who would play doc to you, so it's not necessarily a universal phenomena... i'd imagine there are some on this site? But, i don't doubt your experience either.
i've found with myself that some of my desires/needs are a lot more easily accessed than others. For instance, just about any Guy Who expresses the need/desire to breed me will evoke the bottom in me with little to no discrimination. Add what i perceive as mean, forceful or bullying to that, an i shut down and become unavailable.
There's also things like S/m. i have no desire/need for pain, but i have experienced where a Man has gotten me to want a spanking from Him by using the sub parts of me as a sort of door to someplace i would not generally want to go. Using affection and His own desire/need had a mind fuck effect on me and His accessing and controlling those things positioned Him to spank me. But it wasn't the spanking, it was His affectionate expression of His need that got me.
i don't really have a Doctor/patient fetish, but i would speculate there are similar dynamics at play. That maybe with some accessing other parts of their desire/need could over arch the gender part? Even there, i think it would never be universal, but i suspect some of those guys could be seduced using some of their other needs/wants to get there?