I don't know how to communicate it other than I have.
Do you agree with those who have said that the DM role, in order to function, requires the other players to give that person special trust and respect?
Do you think respect and trust require some amount of justification, demonstrating that they are appropriate, or are they simply something demanded?
Do you believe it is possible for someone, even if they intend to only do good things, to behave in such a way that erodes trust and reflects a lack of respect?
It's not the inequality of the situation. It's that any situation where it is understood that trust and respect are needed, but one's behavior unilaterally dismisses or ignores the desires and interests of the other(s) in that relationship, that behavior erodes that trust and respect. To do so explicitly for no reason other than because of one's own desires and interests IS a rude act, pretty much regardless of any other details. It doesn't matter if no negative intent is present, you don't have to intend to be rude for an act to be rude. Again, you have explicitly disclaimed ANY other possible reason, so practicality isn't a concern; it's not that something couldn't work, nor that the DM isn't up to the task, nor even that money or time investment is a concern. The ONLY reason here is "because that isn't my taste."
What other truly personal relationship in existence (that is, not like the relationship between an employee and an employer, or between commanding and subordinate officers, etc.) affords its participants an absolute, unilateral, and completely beyond discussion ability to nix the non-abusive, non-coercive interests of the other party? If I did this to a significant other, they would rightly feel hurt and angry. Even a parent to a child; remember, practicality can't be a consideration by your own admission, so it isn't a "no, you can't eat the whole bowl of Halloween candy" situation, nor a "we can't afford to go there" situation. This is, "Can we go to Taco Bell or something similar instead of McDonald's next time?" "No, I don't like that kind of food, so we will never eat it, and we will not discuss it further." How is that NOT disrespectful even to a minor child? How is that NOT treating another person's desires and interests with disdain?
Firstly: A person's manner and tone are fundamental, here. They cannot be extracted from the situation, because doing this thing IS demonstrating a manner and tone. You keep trying to silo these things apart, as though one can speak of a pure behavior in isolation without the impact and meaning that behavior will communicate. You can't do that. It's not possible. Particularly when the behavior in question is one of the things I would consider most directly and explicitly demonstrative of such a manner and tone!
As for the three points, I don't think the DM is demonstrating a lack of trust, I think the DM is demonstrating that the trust they ask for isn't deserved. Trust is a thing both earned and given--which is to say that it may be taken back, or it may be squandered, or both. Any DM that unilaterally, absolutely, and obdurately dismisses a player's genuine, non-coercive, non-abusive interests or desires is squandering the trust they ask for from their players. I do think the DM is demonstrating a lack of respect, as I said. It is a necessary, but not sufficient, condition for respect: taking another person's views into account, listening and striving to understand, even if you ultimately don't do things as they want them done. Respect does not require submission, but it does require being neither unilateral, absolute, nor obdurate. (By listening instead of dismissing, one ceases to be unilateral; by striving to accommodate, one ceases to be absolute; by allowing fair hearing and possible-but-not-guaranteed persuasion, one ceases to be absolute.)
Frankly, for me, the selfishness isn't an overriding concern, though I would lie if I said it wasn't one. The DM role implies highly personal involvement. I expect self-interest there. But I expect it to be "enlightened self-interest," which goes beyond petty selfishness and at least somewhat into a more nuanced, delayed/partial/abstract gratification. I mean, as a very simple example, the DM doesn't get the player-side rush of winning or overcoming, their enjoyment comes either by proxy, or from the slow unfolding of a grand thing: delayed and abstract gratification right there. This behavior, excluding things purely and solely because the DM finds them aesthetically displeasing or the like, and for literally no other reason whatsoever, and refusing to entertain even the smallest discussion about it, smacks of petty selfishness.
Ah, with this last point, you have inserted an intent now, have you not? "Denying them game elements." That's no longer just a behavior, that's a judgment. You are, quite clearly, implying a sense of entitlement on the part of the players--which I have never mentioned nor is it even relevant to my point. I have, repeatedly, said that it is perfectly fine for a DM to veto options. I even did so in the post you quoted (albeit in an edit, I admit.) If I were to buy into your phrasing (which I don't, just to be clear, I'm just granting it for the sake of argument), my point would rather be stated as, "The DM is failing at maintaining a relationship with the player(s) by denying them game elements unilaterally, absolutely, and obdurately," meaning "a denial that is completely one-sided, lacking any limit or restraint, and completely beyond discussion or persuasion."
I don't know about you, but if a DM behaved toward you in any other context by being completely one-sided, exhibiting no restraint, and being completely opposed to any form of discussion or persuasion, would you feel they were maintaining the DM-player relationship? If so, I'm very curious what you think a healthy DM-player relationship is. And if not, I'm curious as to why this one special situation is different--particularly when it's one that matters (on both sides, as this over-3000-posts thread shows) to a lot of people.
Except that it has everything to do with it, because in this case the desires and interests are about what things are available for the imagination to pursue. Sure, it's not "about imagination" in the sense that I didn't specifically use that word. But it is "about imagination" because we're talking about an absolute, unquestionable, and completely one-sided declaration about what can be imagined.
I honestly cannot believe that the existence of dragonborn "deprives you of fun." I literally, truly cannot believe that you suddenly stop enjoying being DM because one scaly person exists. For exactly the same reason that I cannot believe, even as an ENORMOUS fan of dragonborn, that it is completely impossible for a player to have fun if they aren't allowed to play dragonborn.
And even if it does? Now we're back where we started. Your fun is more important than someone else's fun--in fact, so much more important that you get to unilaterally, absolutely, and obdurately reject their fun.
So who's in the wrong here, the player trying to deprive the DM's fun by asking to play a dragonborn, or the DM trying to deprive the player's fun by not letting them play one? They're both people trying to deprive someone else of fun, by your metric. I reject the notion that the DM's fun is so inherently more important that a unilateral, absolute, and obdurate dismissal is ever the appropriate answer.
Ahhh, but now you're stepping back from depriving fun, aren't you? It's now "negatively impacting"--meaning differences of degree are relevant, meaning the possibility of mitigating the impact with some other, positive impact is possible. It's no longer a black-and-white "tieflings = fun is impossible" (or whatever race you wish; I am trying not to harp overmuch on dragonborn). And this further conversation...here, let me put the most emphasis I'm allowed to...
IS EXACTLY WHAT I'VE BEEN CALLING FOR.
Finding a compromise MEANS the player and DM talking to each other. NOT being unilateral: each side conversing with the other and coming to consensus. NOT being absolute: the DM and player striving to meet in the middle. NOT being obdurate: both sides, and again I cannot stress this enough, BOTH. BLOODY. SIDES. being open to persuasion and alternative approaches. It may not work. It may fail; that is okay, successful compromise is never guaranteed. But what should be guaranteed is the EFFORT to see if one can be found, and to find it if it can.
I have literally never deviated from this position in the entirety of the thread (though I admit that having to discuss my ideas did refine them over the first, I dunno, couple dozen pages.) I have repeatedly said these exact words, over, and over, and over, and over. And each time I do, I'm challenged, because it's somehow not enough for these DMs. Because expecting to be able to have a conversation (even if it doesn't end up working) is, apparently, player entitlement.
I have gone up to bat against those who call it being a "control freak" to say no (not just Loverdrive). I have repeatedly advocated restraint and giving those who favor hard restrictions the benefit of the doubt. And yet I am repeatedly construed as taking an incredibly extreme position myself, or else I'm given responses like the ones I have repeatedly quoted (e.g. Maxperson's own "the DM is the Ultimate Authority," Zardnaar's "my house, my castle my game," and other responses from those who can no longer respond in the thread). What am I supposed to think when I have actively worked to be fair and accommodating, and then people throw this kind of thing in my face? What am I supposed to think when every time I call for a more respectful conversation, or a conversation about topics that are more interesting and positive (such as "how DOES one 'say no' in a respectful and positive way?") are rebuffed, insulted, or completely ignored?