Only to a point. By the same token, it's also sometimes your job as DM to say "you can't do that".
I agree - to a point. When it comes to real-world issues, no, the GM shouldn't say that... because it's a GAME.
If we're playing a game set in real world Europe, and a player wants to be a black knight templar? Fine. An openly gay nobleman? Sure. A woman knight (and not a "dame", either)? Bring it on. Just ask the player "do we want to make this an issue - or is it just a facet of who your character is?"
Either way, respect it and move on. Some things aren't worth getting in the way and saying "no" to. I'd like to point out that the most historically accurate RPG I won, "Pendragon", even recommends doing this when it comes to female PCs.
Or you could say "No, they're all male; but there's the Cavaliers of Terenia among whose numbers no male has ever resided - let's start there." At which point you (and maybe the player) continue inventing the Cavaliers of Terenia, which didn't exist anywhere until you dreamed them up while uttering that sentence.
I can get that approach. But again, when it comes to some pretty large facets of a character - why can't we just say "Well, most are male. But you could be one of the few females in there."
What's the harm in changing things up? Making an exception? Our world is full of weird inconsistencies. D&D is a game, and saying "no" to a player who wants to engage in your game, based off things like ethnicities, gender, and sexual preference, just seems wrong to me.
I mean, I get it if, say, you have an all-drow order and the dwarf PC wants to be a member. Then maybe "no" isn't an answer. But saying "you're a girl, and this club is no girls allowed" is just too close to our own recent history for it to be something I want to see reflected at my table. Ever.
That said, my biggest "you can't do that" moment was when an established player in a previous campaign came to me-as-DM wanting to play a full Vampire as a permanent character in a mid-level adventuring party where said Vampire would be, to say the least, vastly overpowered. Fortunately, I realized that ju-ust might be part of the motive - to passively end up with the baddest badass in the game and then quite likely end up trying to force the party to do its bidding - and that it wouldn't end well. Hell, it wouldn't even start well. So things very quickly got to Banhammer 1, Vampire 0.
Fully agree with this. But that's a game balance/mechanics issue, and nothing to do with gender and engagement in the game.
(as a side note, I had a player want to be an awakened bear recently. I inititally said "no" because it seemed silly, but changed my mind, used half-orc statistics, and went from there. And then, because I didn't want every second NPC reacting to it, decided that a lot of animals had been awakened recently, and that the PC was far from unique. And suddenly, huge doors have opened in my campaign world. Good times).
Lan-"licking my chops at the delicious rivalry possibilities should a Knight of Xerexington and a Cavalier of Terenia ever find themselves in the same party"-efan
Yeah, that'll be fun. Arguments over whether the saddle seat should be left up or down will rage on endlessly!
