Sheldon's struggle with gender and sexuality were central to her life and her stories. As a young woman in the 1930s, Sheldon found her options (largely, marriage) limiting. She questioned her sexuality and felt a strong attraction to women that she never really pursued due to social taboo. She certainly addressed issues in her stories that no woman could have written about during that time and been taken seriously. It was only under her male pseudonym that Alice could write freely and receive the praise and respect her writing deserved. Sheldon later said of the identity revelation in an interview, "A woman writing of the joy and terror of furious combat, or of the lust to torture and kill, or of the violent forms of evil--isn't taken quite seriously.... I think that for all of us the sense of being in contact with something that has the potential to do--or maybe (wow!) has done--real evil, gives a little thrill to reading. Some people seem to have projected that onto Tiptree. Maybe I did a little too. So to write on as a toothless tiger was shaming."*
*The quote in this article comes from Biography Resource Center, which you can access from home with your library card. This resource far outshines Wikipedia because the articles are written by respected authorities on the individual and come complete with excellent bibliographies.
Crown of Stars
James Tiptree, Jr.: A Life of Alice Sheldon
Out of the Everywhere and Other Extraordinary Visions
Up the Walls of the World
The Very Best of Fantasy & Science Fiction: 60th Anniversary Anthology
Includes "Women Men Don't See", one of Tiptree's more famous stories