Basically, Brun's ethnicity shouldn't matter. He's defining her by her racial background and, at the very least, that is a bit dehumanising.
I get where you're coming from, but that makes quite a few assumptions about the asker's motivations. Sure Paco here is just interested because he thinks her perceived exoticness is sexy (which is blargh of him), but what about someone who asks out of simple curiosity, or just honest desire to know more about the other person?
Is it fair to suspect racism, dismissal, or even such strong a word as dehumanization, just from that kind of question?
Not an American and I'm pretty much as white as they get. So offering an outside perspective - you'll do well to listen to people who have actual experience with race-related issues. I'm both talking about this in theoretical terms, AND might get things wrong or miss something.
That being said:
You're making two awfully big assumptions here, even if you're not stating them explicitly. Both are, in my view, wrong.
The first is that your intent makes things OK that would not otherwise be OK. Specifically, that if you don't mean something as racist, then it isn't racist. This can be true, but doesn't have to.
Note that the question about ethnicity is almost always phrased as "where are you from?" or similar. It's about ethnicity, but it's about ethnicity in a very specific way. It assumes that someone not looking a certain way is not the default, and therefore worthy of scrutiny. In other words, non-white people are strange enough to warrant questions if your intent is benign.
This has implications, and pretty racist ones. American culture is theoretically built on individualism and, in modern times, equality. Trying to pinpoint someone on the world map implies that they're not actually American (note: you ask specifically because you want to know where they're "from". You're immediately jumping to a conclusion that even if they were born in the USA, they're not "really" from here. That their ethnicity means they're also from somewhere else). White people don't usually get asked that. From what I hear, that almost never happens. This is inherently racist. Non-Hispanic white people in the US are at 63% and the percentage is decreasing. But it's assumed a white person is not from Germany, France or Russia. The default assumption is that they're "from here".
It's also a matter of connotations with the question. Racism exists in America and shaped its history, and when you say something, you share whatever baggage the words you uttered have. You don't get to claim ignorance. Asking a loaded question just because you're innocent in intent doesn't make the question any less troubling. Today's comic actually illustrates this pretty well, by Brun being clueless about the implications of the question. See, nobody's blaming Brun. But we know from the comic that she often asks extremely improper questions. She can be excused to an extent and it's often played for comedy, but it's generally understood some of the things she says are generally considered rude. That's WHY she stands out and her questions register as unusual and/or funny. They're things to say that are improper, usually.
Which brings me to the second way I think you're missing something important. Such a question would be inappropriate even in the ideal fantasy world where racism doesn't exist. It's a question that comes out of nowhere. You say it may be asked just to "learn as much as possible about a person".
Except, no it isn't. You see the way someone looks that you don't even know, and you ask them about their looks. This is inappropriate for the exact same reason you don't ask an obese person "so, why are you so fat? Is this a health thing or...?". It's invasive, and inherently judgmental. Like the person has to explain their health history (and with race, genetic history) to you? It's not a casual, get-to-know-you-better question. You don't ask strangers such questions, because it's rude even if SOMEHOW racism is not a potential factor (and it always is, see above). You're immediately making the judgment that you need to know this physical thing about a person, and that's a good conversation starter? Why, exactly?
See, theoretically asking a person about their ethnicity might be acceptable. Either because you need to know that for a legitimate reason, or you know the person well and you're asking about their family history because you're genuinely curious. But if you're not close friends with someone, in USA culture you generally don't pry into their private life. You don't necessarily ask about their childhood, or parents, or other such things. Asking about ethnicity is like asking about family history, and nobody does that casually to a stranger. And also, it raises the question about why you thought this was the first question to ask.
So to sum up, when you ask about ethnicity:
1) you're exoticising a person and treating them like a curious animal, not a person.
2) You're unaware or you decide to ignore the history of racism in the US, which is there whether you claim innocence or not.
3) You ask a very personal question that is just not that appropriate irrespective of racial baggage.
4) You focus on someone's looks and you pry into something that you honestly have no business being interested about if the two of you aren't very close.
5) You, consciously or not, paint being white as a default that someone not fitting has to somehow explain or have stories about or elaborate on. Something that doens't happen to white people usually.
6) If you frame the question as "where are you from?", the unspoken question is really "you're not white, so there's a high chance you're not American even if you were born here. It's different and requires more scrutiny and reframing even if nothing about you otherwise indicates that's actually true".
7) If you're immediately curious about someone's ethnicity and you're not, like, an anthropologist or geneticist (who still shouldn't ask overly personal questions), it's debatable if it's actual curiosity. Why would you ask THIS specific question of a person? You're implying it's OK for you to pry just because you're "curious".
And note - all I've written is just the tip of an iceberg. Anything ethnicity related has connotations and hidden meaning and entanglements that reach back hundreds of years even if you're being conservative. By necessity, I can't explain the history of race relations and colonialism in one post, even if I knew it all
