....
My own suspension of disbelief matters to me while I'm DMing--I am tempted to say I can't DM without it. That's part of the reason I'm homebrewing the world--I have a harder time managing suspension of disbelief in the worlds others create.
So, the folk I don't allow, I don't allow because they don't make sense to me, in the world I'm making. I am not intending to accuse any player who wants to play something I'm not allowing of anything worse than possible tone-deafness. There is no "purity of the world" to be violated, there is no judgment of playstyle.
This nails it imho. I could have written it myself.
I put
a lot of work into making my own world out of whatever published setting I use as a baseline.
This includes but is not limited to: Filtering NPCs, available races, classes, equipment, spells, items, deities, monsters, locales, parts of the official canon, structure of the multiverse (aka which planar configuration), economics (what kind of coinage and pricing), technology level, etc. etc.
I try to meet players expectations, and I talk upfront about the relevant things like playable races. If a player comes along and puts some constructive argument on how a race / class I did not plan in would be meaningful in this world then I am open to change that.
But if a player comes to the table and insists on playing some exotic race or a class which has no background in my personal view of the setting e.g. paladin in darksun (Or ravenloft ;P, yes you can play one if I master a ravenoft game, for about an hour or so if you are real good ^^)
then I refuse.