I'm currently reading this book, which was published in 1986. Her argument about the development of patriarchy is interesting and thought-provoking, but what I want to share here is her definition of terms. Comparing this text from just over 30 years ago to the current state of identity-based 'feminism' really is saddening. I don't think anyone could have predicted that the use of gender as a euphemism for sex would eventually result in people arguing that gender is innate and valuable, and sex irrelevant or even non-existent. 
Lerner states:
Women are a Sex. Women are a separate group due to their biological distinctiveness. The merit of using the term is that it clearly defines women, not as a subgroup or a minority group, but as half of the whole. Men are the only other sex. Obviously, we are here not referring to sexual activity, but to a biological given. Persons belonging to either sex are capable and can be grouped according to a broader variety of sexual preferences and activities.
Gender is the cultural definition of behaviour defined as appropriate to the sexes in a given society at a given time. It is a costume, a mask, a straitjacket in which men and women dance their unequal dance. Unfortunately, the term is used both in academic discourse and in the media as interchangeable with "sex." In fact, its widespread public use probably is due to it sounding a bit more "refined" than the plain word "sex" with its "nasty" connotations. Such usage is unfortunate, because it hides and mystifies the difference between the biological given - sex - and the culturally created - gender. Feminists above all others should want to point up that difference and should therefore be careful to use the appropriate words.