We understand what the meaning of the terms name, age, and address when used in applications and résumés, but why is sex used instead of gender?
Definitions from the Free Dictionary :
meaning of gender: The properties that distinguish organisms on the basis of their reproductive roles.
meaning of sex: Activities associated with sexual intercourse.
Broadly speaking, "sex" is the biological equipment you've got downstairs, while "gender" is what's in your head. Wikipedia goes into this in detail, the distinction and whether it even exists is all rather complicated and controversial.
Employers don't particularly care if their employees think they're men, women or somewhere in between (gender), but they may care about how many male and female bathrooms they need (sex).
Of course, to an increasing extent employers don't care at all about either, and in the US in particular it's recommended that you do not list your gender at all in your resume or application.
Merriam-Webster's Eleventh Collegiate Dictionary (2003), which normally arranges its definitions chronologically from earliest occurrence to latest, has the following entry for gender:
gender n (14c) 1 a : a subclass within a grammatical class (as noun, pronoun, adjective, or verb) of a language that is partly arbitrary but also partly based on distinguishable characteristics (as shape, social rank, manner of existence, or sex) and that determines agreement with and selection of other words or grammatical forms b : membership of a word or a grammatical form in such a subclass c : an inflectional form showing membership in such a subclass 2 a : SEX {the feminine sex} b : the behavioral, cultural, or psychological traits typically associated with one sex
So the word gender was first used in connection with grammatical classes and subclasses and only later with sex (in the sense of what the Eleventh Collegiate calls "either of the two major forms of individuals that occur in many species and that are distinguished respectively as female or male esp. on the basis of their reproductive organs and structures"—the earliest meaning of sex). This fact raises the natural question, When did gender in the sense of "sex" arise and become a commonplace way of referring to that difference in male and female forms of various species?
I was surprised to discover that Webster's First Collegiate Dictionary (1908) puts its definitions in a different order, but with the first two definitions labeled obsolete or colloquial:
Gender n. 1. Kind; sort. [Obs.] 2. Sex. [Obs. or Colloq.] 3. (Gram.) A classification of words, primarily according to real sex, and secondarily according to some fancied or imputed quality of sex.
Throughout this entry, sex is used consistently with the following entry for sex in the same dictionary:
Sex n. 1. The distinguishing peculiarity of male or female; the physical difference between male and female. 2. Either of the two divisions of organic beings distinguished as male and female.
The sex, the female sex; women, in general.
As you surely noticed, "sexual intercourse" is not included as a definition of sex in the First Collegiate. Half a century later, in the Sixth Collegiate (1958), the labels in the gender entry had changed, and the treatment of the grammatical definition of gender had become much more elaborate, but gender in the sense of "sex" was still treated as colloquial:
gender n. 1. Archaic. Kind; sort. 2. Colloq. Sex, male or female. 3. Gram. Form of a noun or form or selection of other words (as adjectives, participles, pronouns) used with the noun as a mark of the noun's membership in a distinct class; also, one of the classes, or such classes, so distinguished. In most Indo-European languages (as Latin, English, and German) there are three genders, masculine, feminine, and neuter; in some (as French and Italian) there are only two, masculine and feminine. Some non-Indo-European languages have many genders.
and "sexual intercourse" was still not a listed definition of sex:
sex n. 1. One of the two divisions of organisms formed on the distinction of male and female; males or females collectively. 2. The character of being male or female, or of pertaining to the distinctive function of the male or female in reproduction.
The breakthrough comes with the Seventh Collegiate (1963) which offers "sex" as the first definition of gender (and words the grammatical definition very similarly to the way it looks 50 years later), and which adds a couple of new definitions to the entry for sex:
3 a : sexually motivated phenomena or behavior b : SEXUAL INTERCOURSE
The decision to reverse the order of the grammatical definition of gender and the "sex" definition of the term occurred after the Ninth Collegiate (1983) but prior to the Tenth Collegiate (1993).
Conclusion
During most of the 1900s, at least, sex was the standard term used in the United States to refer to the differentiation between male and female; and gender was primarily a term used in connection with grammatical analysis. The definitions that the OP introduces for gender ("properties that distinguish organisms on the basis of their reproductive roles") and sex ("activities associated with sexual intercourse") may reflect many people's understanding today, but they are not the exclusive meanings of the two words, and it is hardly shocking that some applications and other forms continue to refer to sex in the sense that the OP considers appropriate only for gender.
