Intersex people exist, but about 98% of people can be unambiguously categorized as male or female. This categorization primary has to do with which role they play in sexual reproduction (which evolutionary speaking is why sex exists).
Of the remaining 2%, about 1.8% can be categorized as one or the other but there is some ambiguity, and about 0.2% are truly ambiguous.
There is no "bright line" between male and female. Only a very blurry one.
This is just false. In the overwhelming majority of people (~98%) ALL the physical markers of sex are concordant with one or the other sex. The existence of exceptions does not mean the categories are invalid or meaningless.