See, I don't think D&D (nor much in the way of fantasy in general) cares about the REAL middle ages, they care about the romanticized version that floats somewhere in Jung's collective unconscious. A true medieval world would be very different and radical from the notions we take an romanticize in games and literature.
For example, Take gender equity. D&D is pesudo-egalitarian; there is no official distinction in gender by the basis of ability (maximum strength) or profession (female clerics). Though older D&D (1e) did create an ability distinction between genders (and visages of this lingered on, such as drow favored classes and 2e bariaur racial traits) we accept female PCs are as equally competent at their game role as male PCs.
So D&D becomes a world of female paladins (knights) and clerics (priests); something completely aberrant to the medieval mindset (odd exceptions like Joan of Ark permitting). In a truly medieval world, women would mostly be chattel or breeding stock; with primary functions of home-making and child-bearing. Even nobility rarely improved a woman's role in society; women merchants and scholars were few and far-between. Women would never be allowed to become knights (dame as a title typically referred to the wife of a knight in medieval times) and the Catholic church never would consider female priests. Occasionally, a smart and ambitious woman would rise to power (typically royal blood) but truly powerful Queens are a Renaissance ideal, not a medieval one.
So we hand wave that notion for a better game idea. We accept some females are passive to indulge our "rescue the princess" stories, but we accept capable female warriors and priests. We allow our female PCs rights of land and ownership, do not arrange their marriages, and give them autonomy un-thought of in medieval times. It makes a better story and game.
Much the same is done about religious tolerance, racial equality (both of human races and non-human races), more modernistic notions of coinage, property, rights of man, medicine & healing, science, and crime & punishment. We take our modern world (or our idealized notion of it) and sprinkle medieval tropes on it rather than creating a medieval world.
And that's not EVEN touching magic!
Finally, I'd like to plug A Magical Medieval Society: Western Europe as further reading. It does a good job of trying to create a more "authentic" vision of a medieval world using the D&D structure. A much smaller text of a similar notion is found in 3e' Dungeon Master's Guide II.
Greetings!
Remathillis, I love this.

Yeah, I can imagine how *forcing* lots of medieval prejudices and conventions upon players--especially females--could potentially result in "less fun". However, in one of my campaigns, I actually ran with a number of medieval campaign assumptions for women. One woman player in my group experienced condescending and even outright shocked outbursts from other knights that she would dare to think she could
wear armor and pretend to be a noble warrior.
Yeah.

She got into it with these knights, too, in many heated debates while the group camped at night during their travels together. Man, did she ever get
*hot* at those other NPC knights!

One of the knights even sought to bring charges against her for seeking to--
(1)
Impersonate a knight;
(2)
Fraud;
(3)
Charlatanry;
(4)
Insolence towards a noble;
(5)
Sedition against her betters!
When she got involved with the law, she and the group had a big court-showdown, lawyers arguing back and forth with arrogant, old-bastard judges, quoting different books and philosophy, arguing law, justice, and rights, wow.

Well, her father is a powerful nobleman, so the group's reputation, deeds, and history of faithful service ultimately prevailed, as well as some excellent arguments that they made of both logic and philosophy to save her from anything more serious than some fines. However, it had other implications. When she refused to marry the nobleman that her father had selected for her, well...he started in about how much of a troublemaker she was, how defiant she was, and how insolent and careless she was of the family's dignity, prestige and honor. She went nuts, screaming at her father; he slapped her down, and had her imprisoned inside a tower, sternly telling her that she would remain there until she gained better sense and recovered her good morals, and stopped acting and thinking like some kind of disrespectful, childish tart, and regained her traditional values of being respectful and obedient to her elders, her father, and joined in the marriage planned with the noble.
Yeah. Man, she was arguing back and forth, crying. She was livid--at her father. I mean, not just her character--but her, the player. All very much
in character, though. The group rescued her from her father's tower, and left the area immediately. Much later--after her father had talked with her mother, and so on--and her character had returned from some great and glorious adventures, with a new, noble and glorious knight on her arm as her lover and champion...well, she had another emotional time of reconciliation with her old, conservative,
I-love-father-but-he's-an-arrogant-sexist-bastard--that ultimately resulted in a tearful reconciliation between the two. Yeah, more tears, passionate speeches and declarations of love and some very good arguments not based on what she couldn't do because she was a woman, or what she
shouldn't do because she was a woman--but what she had actually accomplished, what values and honor and dignity and righteousness that she actually stood for, believed in, and championed.
And--how much she lived and breathed for the love and acceptance of her father.
It was very intense. Some of the best game sessions and characterization and roleplaying I've had in all the 30 years I have been playing.
Semper Fidelis,
SHARK