Have you read Anne Fausto-Sterling's The Five Sexes? Written in 1993, it discusses intersexuality and the ways in which our Male/Female system of sex-categorization is insufficient. I was only 10, so I can't tell you how radical a notion that was at the time, but I can tell you that theorists and activists and scientists -- including Fausto-Sterling herself -- have taken the building blocks of The Five Sexes and used them to continue to evaluate and reimagine the way we think about sex.
A Thousand Tiny Sexes is a book project whose aim goes way beyond just five sexes. From the website:
To imagine that there are only two sexes - M & F - is an absurdity. There are at least 1000 sexes. The daily lived reality of transgender and intersexual people (and gay, lesbian, queer and all other people, for that matter) proves this over and over again, yet many people continue to operate as if 'M' and 'F' are the only sexes, the only options, the only expressions, the only goals, the only way ("the way it is").
The ideology behind A Thousand Tiny Sexes is quite radical. They're not just talking about gradiations of female and male; they're looking for submissions that "entirely reimagine sex as a biological, cultural, political, and social category." Really interesting stuff; I'm looking forward to seeing the finished product.
Image: "Found Sex #1" by kanarinka, one of the editors of A Thousand Tiny Sexes.