With edits by ellenor2000.

First of all, in a sexual species,

  • you can have females be XX and males be X (insects),
  • you can have females be ZW and males be ZZ (birds),
  • you can have females be females because they developed in a warm environment and males be males because they developed in a cool environment (reptiles),
  • you can have females be females because they lost a penis sword fighting contest (some flatworms),
  • you can have males be males because they were born female, but changed sexes because the only male in their group died (parrotfish and clownfish),
  • you can have males look and act like females because they are trying to get close enough to actual females to mate with them (cuttlefish, bluegills, others),
  • or you can be one of thousands of sexes (slime mold, some mushrooms).

Oh, did you mean humans? Oh ok then.

  • You can be male because you were born female, but you have 5-alphareductase deficiency and so you grew a penis at age 12.
  • You can be female because you have an X and a Y chromosome but you are insensitive to androgens, and so you have a female body.
  • You can be female because you have an X and a Y chromosome but your Y is missing the SRY gene, and so you have a female body.
  • You can be male because you have two X chromosomes, but one of your X's HAS an SRY gene, and so you have a male body.
  • You can be male because you have two X chromosomes- but also a Y.
  • You can be female because you have only one X chromosome at all.
  • And you can be male because you have two X chromosomes, but your brain is wired in a manner identical to a male.
  • And vice - effing - versa.

Don't use science to justify your bigotry. The world is way too weird for that shit.