If I'm writing a short story (let's say it's fan fiction for "Little Women") about a family in a New England village in the 1860s, images that include non-white people in them are going to look absurd and be ahistorical. Your assumption that the only reason to prompt for an image of an ethnically homogenous family is due to bigotry appears to reflect a negative and pessimistic view of humans. I would guess this view stems from spending too much time reading Twitter comments and forgetting that they are written by a tiny, non-represenative sample of the population that skews towards derangment.
I get that people will be really sad that they don't get to look at your AI generated 1860s white family because Google has made it illegal for all AIs to create images of white people. However I'll do you a solid: I happen to know a couple white families and I'll get them to pose for your short story. Just PM me the description and I'll hook you up.
Sometimes people want photos of families that look like their current reality.
I wouldn't want someone from a visibly different ethnicity when I'm looking for something like this.
Even black communities around me have visibly discernable differences. Where I live driving an hour might be enough. We aren't a monolith anywhere
When you take photos of your family do you rent others of a different ethnicity to avoid looking racist?