Gender was traditionally used mainly in grammar, language, and linguistics contexts to refer to the sex assigned to nouns (especially in non-English languages). For example, the gender of the French noun maison (house) is feminine, while the gender of livre (book) is masculine. Words of the same gender tend to have similar endings, and they affect the forms of some of the surrounding words. Sex, meanwhile, was traditionally the term for males or females viewed as a group.
In recent decades, the meaning of sex has narrowed, and the word is now mainly confined to uses having to do with sexual intercourse and sexual organs. Gender, meanwhile, is increasingly used to refer to a person’s maleness or femaleness. For instance, we tend to say that a boy’s gender is male and a girl’s gender is female. Of course, the term is more complicated than that, and gender identity is not always tied to one’s sex organs. This at least partially explains why gender is now preferred in this extended use; gender denotes identity, which can be fluid and complicated, whereas what sex organs one has is pretty straightforward.
Sex is still sometimes used in its traditional senses. No one considers it wrong, but it tends to give way to gender for the reasons mentioned above and also because gender is considered more appropriate in contexts where sex and sexuality are not to be brought up.
Examples
Disparities: A Growing Gender Gap in Doctors’ Pay [NY Times headline]
There are several reasons why Chisolm’s race and gender overshadow her legislative achievements and political stances. [Gender and Political Communication in America, Janis L. Edwards]
Apparently, the two aren’t letting their babies’ gender be a surprise. [ABC News]
The 1986 EEOL had only limited effects upon gender discrimination in employment and might have created more difficult conditions for Japanese women. [A Path Toward Gender Equality, Yoshie Kobayashi]
The European Court could outlaw the use of gender in determining insurance pricing on 1 March when it rules on the matter. [Money Saving Expert]