That idea is known as “essentialism”: the belief that there are uniquely feminine and uniquely masculine essences which exist independently of cultural conditioning. Both actual (minor) and alleged (major) differences between the sexes have been used to justify inequities and constraints which harm women emotionally, financially and physically.
Even where (and if) such differences do exist, why should such differences justify sexist oppression?
Biological determinism is one form of essentialism which has been used to argue for male superiority for all of recorded history: (wow, reasoning class has taught me well! :D)that men are naturally stronger, smarter, more rational and more trustworthy and thus are entitled to rule both politically and domestically. The more science discovers about biology the more this male biological superiority is shown to be utterly without foundation: for any quality measured there is far more variation among the group of all men and among the group of all women than there is on average between individuals of opposite sex.
A common belief is that while men are physically and rationally superior, women are morally superior. At times influential groups of both men and women, both feminists and anti-feminists, have subscribed to this view. It is equally without evidentiary foundation, and has often been used to give women a sense of power in the role of morality enforcer which acts to support the larger social system of male dominance (and which especially excuses the male sexual exploitations of women as due to a baser moral nature which can’t be changed, but which “good” women have the duty to “tame”).
Masculine and feminine traits have been culturally placed in opposition to each other, and claimed to thus complement each other and result in harmony when men and women are constrained within the accepted sex roles. Masculine roles differ across societies, but are always portrayed as not only different from but also superior to the feminine. Women and men who transgress the boundaries of the accepted sex roles are considered “not real” men/women, and usually defamed and sometimes abused and punished by outraged defenders of normative sex roles. It is this rigid segregrating of masculine and feminine, and the assigning of superiority always to the masculine, that feminism challenges.
It is often claimed that feminists say there are no differences between men and women, by people who tend to condescendingly point to women’s chest area as they “debate”. Rubbish – feminists are, on the whole, not blind. What feminists say is that neither the size of the fatty glands on one’s muscles, nor whether one’s reproductive organs are innies or outies, are indicators of deeper essential differences, and nor such indicators of sexual dimorphism relevant when discussing rights, equity and sexual egalitarianism.