The meanings of words are generally stable over time, but when a shift does occur it can often be attributed to a change of ideology in the culture. This is the case for the fading of the word sex as the traditional designation of the biological category and its replacement by the word gender, which was once restricted to the field of grammar.
The Oxford English Dictionary Online (OED) gives the following definition as the primary one for the word sex: “Either of the two main categories (male and female) into which humans and many other living things are divided on the basis of their reproductive functions; (hence) the members of these categories viewed as a group; the males or females of a particular species, esp. the human race, considered collectively.” A secondary definition reads as follows: “Quality in respect of being male or female, or an instance of this; the state or fact of belonging to a particular sex; possession or membership of a sex.” With regard to persons or animals, the entry supplies the following commentary: “Since the 1960s increasingly replaced by gender . . . when the referent is human, perhaps originally as a euphemism to distinguish this sense from . . . Physical contact between individuals involving sexual stimulation; sexual activity or behaviour, spec. sexual intercourse, copulation. to have sex (with): to engage in sexual intercourse (with). Now the most common general sense. Sometimes, when denoting sexual activity other than conventional heterosexual intercourse, preceded by modifying adjective, as gay, oral, phone sex, etc. . . . The word sex tends now to refer to biological differences, while gender often refers to cultural or social ones.”
It is both interesting and ideologically relevant to note that many foreign languages (and not just the European ones) have borrowed the English word sex in the meaning “denoting sexual activity,” e. g., Russian секс (seks) and Japanese sekusu (セクス).
With respect to the native English cultural development, there is no gainsaying that the linguistic substitution of gender for sex serves to individuate the latter word in its social sense as part of the pervasive sexualization (including that of children in the United States at least) so characteristic of modern culture all over the globe. Insofar as a devaluation of the dignity of the individual human being can be descried in this phenomenon, English as the language that first offered up its linguistic expression can only be reckoned to bear full responsibility.