How is sex classified?
The classification of sex is determined by recognizing a natural division among humans. Sex is female, male or intersex, where intersex conditions are disorders of normal sex development. Thus, sex is primarily described as a binary, i.e. there are two possible categories (female/male). The distinct physical traits of each category are used to identify sex.
Think, doctors use “it’s a boy” for males and “it’s a girl” for females.
Sex is not assigned, but determined by generalizations in classification. This is why although mistakes at birth may be made, they are rare.
Ex) a baby born with an identifiable penis is recognized as male because the penis is derived from XY sex chromosomes and XY sex chromosomes are classified as male
In the above example, the likelihood of a baby born with an identifiable penis actually being female is very low. If the external genitalia are ambiguous, an expert may be called in and genetic testing may be done to determine sex.
In summary, sex recognizes a natural division among humans. The physical traits of females are generally only found in females and the physical traits of males are generally only found in males.
Pictures from http://www.sexualityandu.ca/