2) While it could be argued that there are only two sexes, it's far more difficult than it might seem to create an accurate definition that covers everyone. Sure, male or female works fine for almost everyone...but not everyone. Also, how do you define "male" and "female"? That's a serious question and the devil is in the details. Chromosome type is simply wrong - genotype is not phenotype. So it has to be biological but what, exactly?
Its really very easy to define 'sex' 'male' and 'female'
sex
sɛks/
noun
- either of the two main categories (male and female) into which humans and most other living things are divided on the basis of their reproductive functions.
A man who has had cut his penis cut of, had something visually approximating female genitalia constructed in its place, had breast implants and who is taking female hormones is not a woman (ditto for a trans man) .
A sufficiently advanced science (far beyond our current level) may one day be able to literally change the sex of a human but that's not currently possible.
So this usually ends with the retort about what about (biological) women who can't have children? (and the corollary point for an infertile man)
Well they are still women just women with a defect much the same as one of the defining features of humans is to be bipedal.
Someone born with a defect which leaves them with no or deformed legs which can't be walked on or someone who loses their legs in an accident is still a human just a deformed one or a severely injured one.
A trans woman is not the same as an infertile woman.
So finally we come to intersex people (who account for a very small percentage of people).
Now of course anyone who remembers much of their biology lessons might recall that in a rather crude sense all humans in the womb 'start of' as being of a similiar (female) body plan and after that develop based on hormones (hence male nipples).
And for the small percentage of intersex people this process doesn't complete as it normally should leaving them with some biological indicators of both sexes be that physical, chromosomal or other.
So a third category for intersex people may be appropriate. But trans people aren't generally intersex. A pre op trans person would normally, by all objective measures, be able to be categorised into the sex that aligns with that on their birth certificate.
Personally I suspect (that at least in some cases) that being 'trans' isn't so much about actually thinking you are of the other sex but rather a phillia for becoming a caricature of the opposite sex. Which may account for some information suggesting 'trasitioning' doesn't seem to improve mental health outcomes and may help account for why a lot of trans women seem to adopt a rather accentuated feminine persona.