Two articles illustrating the fact that "sexual characteristics exist on a spectrum—not as a binary":Sex Isn't Chromosomes
Sex identity is far from a simple binary among some "lower" animals, such as fish that change sex during their lifetimes (like Robert Heinlein's Martians). Another example: A male platypus has ten sex chromosomes! Among human beings, there exist XXY men and XYY men. The article discusses science historian Sarah Richardson's SEX ITSELF, which traces the development of scientists' determination (erroneous, she declares) to attribute all distinctions between the sexes to X and Y chromosomes. It's estimated that about one percent of people fall into the intersex range, with chromosomes, hormones, and/or genital anatomy that don't "match." The assumption that simple, binary sexual dimorphism between male and female covers everybody doesn't correlate with the realities of human biology.Sex Redefined
This article delves more deeply into issues of gender identity in intersex people and also explores chimaerism and mosaicism, in which some of an individual's cells contain DNA of the opposite sex.
These biological phenomena can contribute to speculation about alien sexual biology—will it necessarily follow the "binary" pattern we're used to? Suppose the various functions "normally" divided between male and female among terrestrial mammals—producing sperm or ova, gestating the fetus, feeding infants—are distributed differently in an alien species. Sperm-producing males might get pregnant (like seahorses or the men in the TV series ALIEN NATION) or lactate; three or more sexes might be needed for fertilization (again as in ALIEN NATION); there might even be multiple sexes dividing those functions among themselves.
Margaret L. CarterCarter's Crypt