A researcher claims that because biological sex characteristics are consistent across human populations, the classification of people into two genders, male and female, is a universal and natural system. Based on an understanding of different cultural frameworks, what is the primary flaw in this researcher's reasoning?

A traveler from a society with a strict two-gender (male/female) system visits a culture that has, for centuries, formally recognized a third gender category with unique social roles. The traveler concludes, 'This third gender is just a confusing deviation from the natural biological standard of two sexes.' What fundamental error in reasoning underlies the traveler's conclusion?