But knowing someone was from America, in fandom, I assume they are white. Without any prompting to the contrary, I'd also assume straight, female, and middle-class, college-educated. Without knowing someone's nationality I'd assume American, maybe British, certainly spoke English as a natal language. In this particular case I've known this person for a while (I don't want to specify who this is, both because it's incidental and because I don't have her permission to share the details off Tumblr) and so I actually do know some other indicators.
But I've been thinking about this issue since she made this passing comment, because it strikes me that this is how privilege works, how assumptions work. I know at an intellectual level my assumptions probably aren't accurate in every particular, even for the majority of people. For one, I wrote in the Tolkien fandom and currently write in the Sherlock fandom, where I have every reason to assume people are British (that country produced both canons, and the books are probably even more popular there than here, after all). And so many fans in Sherlock (+ Tolkien for that matter) are German - my only excuse really is I couldn't use a second language with half the fluency they manage, and I actually only spoke German until I was three! But I don't think so clearly as that. The women thing is probably a safe bet in fanfic circles but doesn't hold true across the board.
Interestingly, this is why I've never been a big fan of the idea that a society where you never specify race or gender or whatever is one where you don't have to worry about racism or sexism. Because I think a lot of people make assumptions that if the (for example) race isn't specified, the character is white. (This played out rather interestingly with the Hunger Games movies, where some people were quite upset to discover Rue was played by a black actress - never mind the fact that in the canon she's described as black!) I try my hardest not to be sexist or racist or any of the rest, but no matter how hard I try I will have a "default" that I assume unless told otherwise. Famously, importantly, in the Sherlock fandom (at least the Tumblr contingent) there's a lot of talk about the way people assume Sherlock and Molly are in a romantic relationship but resist the idea of John/Sherlock when there's a lot more suggestive looks, because we will assume a character is heterosexual (and that a man and a woman who are close have to be romantically interested in each other), whereas those same assumptions don't really apply between two men. We will assume a man or woman for that matter is straight until we're told otherwise.
I'm also thinking about this in the context of some comments Moffat made re: Doctor Who, about how he wouldn't cast a female as the Twelfth Doctor just because it's a political decision, he would do it when a particular female actress felt "right" for the role. And at some level that seems right to me. I wouldn't want an affirmative action hire, kind of like how I wouldn't want to be accepted at an academic conference just because they needed a woman on a certain panel. (This is something discussed in academic circles, whether you should agree to present at a conference where the headliners were all white men, and I balked at something about that assumption.) The problem is, if you're used to thinking of the Doctor as a white man, no one but a white man is ever going to feel right as the Doctor. Just as the companion will (past history being what it is) probably always be either a youngish, prospect-less woman or the love interest of the same (in the case of Mickey and Rory). Sarah Jane Smith would never fit in by the time we see her in New Who. So I understand why Moffat feels he couldn't choose a woman Doctor because she's a woman, but at the same time, the unexamined "gut instinct" can just as often lead you wrong.
I think the best I (we) can do here is be aware of this bias. Yeah, I wouldn't have ever treated this friend any differently if I'd assumed she was Asian-American or black or Hispanic or purple-skinned with yellow polka dots, because I'm not actually a racist, but assuming people are white kind of holds them up to a standard or a way of approaching them that doesn't quite fit them as well in subtle ways, that expects them to act by a framework that's more organic, that comes more naturally to people who actually meet the assumption. I mean, take the current situation with the girls kidnapped in Nigeria, for example. As someone without any particular ties to Nigeria, I may be outraged but in an abstract way. For me it's just a news story, it's far away. Compare that to someone who's actually from Nigeria or from the kind of situation where this kind of thing could happen. (My brother-in-law, for instance, is Lebanese, and I know how news stories involving Israel affect him very differently than they do me, because he has very specific memories of attacks made on his neighborhood by Israeli arms in a way I can't have those experiences so whatever I think of Israel's position in a certain conflict, agree or disagree, my reaction will be much less disinterested than his would. Rightfully so.) Now, if I just know your online handle, if I assume you are from mainstream white culture, I will assume you will react to that news story like I do - even if perhaps you shouldn't because you're not from that culture, because the story actually hits closer to home and so of course you're going to react to it differently. I can't get past those assumptions and the other thousands of assumptions I make about characters or people very easily, and it's not always a matter of asking the right questions and getting better information. But I can at least be more humble and skeptical about my inferences, and be realistic about how accurate my suppositions may turn out to be.
Or something like that. This is me pre-coffee, so this may not actually make a lot of sense. But it's on my mind today, and it seemed worth sharing.