they just have one sex! physically and literally....
K.A.'s official canon final awesome answer...of doom.
(Side note: Aww. KA doesn't like guns? Maybe she's never been shooting. I was hooked after my first five rounds.
)
OK. Gather 'round for a biology lesson. Sex is weird, even on familiar old Mater Terra. Here's how some different forms of life shuffle genes among terran species:
Plants alternate generations: half the generations have twice as many chromosomes as the other half. The plants fewer chromosomes are called gametophytes; the ones with more, sporophytes. Both generations are multicellular. One of the generations is dependent on the other for development (and is physically much smaller). In most plants, the sporophyte is the larger, longer-lived, dominant generation. However, the opposite is true of mosses and ferns, where the gametophyte hosts the sporophyte. Gametophytes always have an inherent sex (mating type), but sporophytes often do not: they'll produce both sexes of gametophytes, or they'll change from producing one to the other with age. And many plants ignore sex most of the time and just clone themselves all over the place: clover, strawberries, grass (the kind that grows in steppes and meadows, silly), etc.
Fungi are similar to plants, except that the haploid generation is dominant, "sex" is called "mating type," many haploid fungi can change mating type (gametophytes can't do this), and sexual reproduction usually only occurs in response to environmental stress. (why do I get the feeling that will be taken out of context?)
Protists are just strange. Unicellular. Multicellular. Sexual. Asexual. They confuse the professional biologist, so, I'm going to move right along.
Animals alternate between multicellular diploid generations, and unicellular haploid generations. Except for social insects (ants, bees, etc), which have multicellular haploid generations. The haploid cells (gametes) are always sexed (like plant gametophytes--except unicellular). The diploid animals, though, have a wide variety of sex determination systems.
Genetic determination
XY: (Mammals, except platypus): female, unless the male-marking Y chromosome is present.
ZW: (Birds, some insects, some fish): male, unless the female-marking W chromosome is present.
X0: (grasshoppers, crickets, ****roaches): Females have two X chromosomes; males, one.
Haplodiploid: (bees, ants, termites, bark beetles): A variant of X0 in which males are haploid (having one of each chromosome, not just X) and females are diploid.
Environmental determination
Incubation temperature: (alligators, turtles): sex is somewhat random, but influenced by temperature. Yes, that's right. Weird hunh?
Wolbachia: (some insects): sex is determined by a parasitic bacterium. And I thought the last one was strange!
Social variables: (many fish): sex changes to match behavior.
And so, with all this strangeness, on our own planet, aren't you glad KA took the easy way out?
