As far as I know my ancestry (and I really don't know it all the way) I'm generally of Greek and east European heritage, but my hair is so so so curly, so I sometimes (often) get asked by the ignorant "what are you" and it's so cringe worthy because I can tell that to them I'm an "acceptable deviation that makes me look unique but still very white, thus safe." I usually just tell them I don't know, in order to discomfort them (white people tend to be sort of obsessed with ancestry and purity and genes and stuff so I like to make it clear I don't know and don't care to get rid of them).

However, there's a girl I work with who's "a mutt" (her words) and has the same curly hair but darker skin, and she once asked me from genuine curiosity if I was mixed (I think she was just sort of glad to see someone at work who might have been something other than boring old white, and perhaps share cultural heritage and understanding with her). I was thrown off and gave my usual "I don't know" response, which had a completely different context and effect to her than it did the usual white people who ask me. I felt guilty for telling her I didnt know (and thus leaving the question of what we have in common re genetics a mystery) when I did know my ancestry and heritage probably didn't cross much with hers. I eventually brought it up again and told her I was a mix of European dissent and had the vague idea I'd gotten my hair from the Greek ancestry, she thought it was funny and we moved on.

It all really depends on where the questioner is coming from. It's pretty clear to me that white people tend to ask because I look unique in an acceptable way (and if they could, they'd take all the 'cool' or 'beautiful' traits from other groups and keep their white skin/superiority smh) but for my coworker friend she was just curious and looking to see if we had commonality in culture or ancestry out of genuine interest and friendship-making.

I don't think we can equivicoally say that asking about a person's heritage or gender/sex/sexuality is bad, but it definitely depends on who's asking and why.