Conjugant organisms, and perhaps hose couplings, have the attribute of sex; nouns and pronouns have gender.

The misuse of gender in place of sex, born of a misguided quest for a euphemism for the latter, has led to general confusion between the two, contributing, for example, to growing resentment of the conventional use of pronouns of masculine gender to refer to persons of unspecified sex. (See THEIR, etc.)

One problem is that gender classification is relatively unusual in English. The French feminist does not eschew the pencil (denoted by le crayon, a masculine noun) in favor of the pen (la plume, feminine) because gender assignment is commonplace in French and well understood by its native speakers to be substantively unrelated to sex. But to an American monoglot, it remains a mysterious notion, not easily freed from sexual connotations.

On the other hand, the current use of gender as a term of jargon in certain academic circles is difficult to ignore. In an attempt to gain a lay understanding of this usage, I paid a surreptitious visit to a neighborhood New Age book store, where a random sampling from the "Gender Studies" section revealed the following:

Gender refers to the cultural meanings, social roles, and personality traits associated with sex differences. This is the social (as opposed to biological) aspect of being a man or a woman. ... These terms (sex and gender) do not necessarily overlap in reality. Men can be feminine and women can be masculine, and most people do not conform entirely to either designation. *
Further reading confirmed my suspicion that the concept was motivated by a desire to minimize the significance of sex as an incidental biological circumstance:
The great advantage of the sex/gender distinction was that it enabled feminists to challenge the `naturalness' of gender divisions and to theorize them as variable. *
Of course, this perspective is not unique to the feminist community:
Usually defined in lesbian and gay studies as the psychological counterpart of biological sex: one may identify oneself as biologically female but claim a "male" gender, or vice versa. Queer theory, on the other hand, views gender as an artificial social or cultural "construction" or a "performance" one learns as part of developing an individual identity. to challenge the `naturalness' of gender divisions and to theorize them as variable. *

Queer theory indeed.

One interesting difference between sex and gender, viewed medically, is that the latter admits a nonsurgical cure:

Writer and scientist Martine Rothblatt, who describes herself as a "transperson," has advanced the hypothesis that there are at least as many possible genders as there are shades of color and that people should be free to select a unique gender identity and change it at will. *
But the point to be made here is that regardless of whether one is interested in promoting this New Age usage or merely in preserving established meanings, it is simply an error to equate gender with sex. In particular, the following, which is especially jarring in view of the source, is indefensible:
But with the ability of modern techniques to determine the gender of the fetus, sex-selective abortion has become common in many countries. *