late 14c., "males or females collectively," from Latin sexus "a sex, state of being either male or female, gender," of uncertain origin. "Commonly taken with seco as division or 'half' of the race" [Tucker], which would connect it to secare "to divide or cut" (see section (n.)). Meaning "quality of being male or female" first recorded 1520s. Meaning "sexual intercourse" first attested 1929 (in writings of D.H. Lawrence); meaning "genitalia" is attested from 1938. Sex appeal attested by 1904.
Sex drive is from 1918; sex object is 1901; sex symbol is 1871 in anthropology; the first person to whom the term was applied seems to have been Marilyn Monroe (1959). Sex therapist is from 1974.