"Ethnic preferences" are just preferences for select traits that are present in one or more ethnicities. For example - If you're into thicker, taller guys with lots of facial hair you might not be a fan of the common latino or asian type.
Or the alternate explanation - culture told you to not get a boner when you see latino or asian guys.
You see my examples as absurd, but it's exactly the same as the mustard example

If you like strong, stingy flavors you'll like mustard. If you like sweet, tame flavors you'll like ketchup. It's not rational, it's not deliberate, it's not a choice, it's not cultural.