On ne nait pas femme; on le devient -- Simone de Beavoir
At its simplest, the distinction between sex and gender is between a physical difference and a cultural difference. Gender is the mapping of socially and ideologically important distinctions onto biological differences between the sexes. (see also sexuality.)
The distinction between sex and gender becomes important in arguments that lean towards social constructionism, in which gender is given more attention, and is presumably more open to change, than sex. Social construction can change over time and space. Feminism asserts that gender is a fundamental category within which meaning and value are assigned to everything in the world, a way of organizing human social relations. In a further twist on the relation between culture and nature, Brian Massumi calls gendering the process by which a body is socially determined to be determined by biology.
Many of the categories that have been the object of feminist criticism are ones that rehearse the male / female gender polarities, with dominance given to the masculine. see distinctions
For Sandra Harding, in The Science Question in Feminism, "An adequate theorization of gender will always lead us to ask questions about the interactions between gender symbolism, the particular way in which social labor or activity is divided by gender, and what constitutes gendered identities and desires in any particular culture." (p. 56) These three modes of gendering often, but not always, act in concert. "As a symbol system, gender difference is the most ancient, most universal, and most powerful origin of many morally valued conceptualization of everything else in the world around us." ..."Once we begin to theorize gender -- to define gender as an analytic category within which humans think about and organize their social activity rather than as a natural consequence of sex difference, or even merely as a social variable assigned to individual people in different ways from culture to culture -- we can begin to appreciate the extent to which gender meanings have suffused our belief systems, institutions, and even such apparently gender-free phenomena as our architecture and urban planning." ( p. 17)
As Simone de Beauvoir pointed out, Masculinity requires the conception of woman as "other." Central to the notion of masculinity is its rejection of everything that is defined by a culture as feminine and its legitimated control of whatever counts as the feminine. (Harding, p.55)
What it means to be a man is, in part, to share in masculine control of women. The most basic male interest is to make sure that he knows who his offspring are. Females do not share this anxiety. Their most basic interest is to see their offspring survive long enough to reproduce. In patriarchal societies -- the overwhelming majority of human ones -- male lineages need to be known. Women might have an interest in deceitful claims about the paternity of their offspring. Higher status males want nothing of this.
Beauvoir claims that "woman is made, not born." (see transcendence / immanence) Of course, in many cultures, men are "made" as well.