In most jurisdictions only two genders are recognized by law. However, there are several countries such as Australia, Germany and Pakistan which have passed legislation to recognize a third gender category besides man and woman. In Australia and Germany people can be assigned “undefined”, whereas Pakistan has given legal status to their existing practice of “Hijras”.
Some people believe that biological gender is one’s “true” gender. Only what is meant with biological gender? Genetic or phenotypic gender? From a genetic perspective some one with two X-chromosomes is female, and one with one Y and one X-chromosome is male. But there are people who are XXY or X0 or some other combination. Further some genetic males, have full female appearance complete with full female sex organs (so-called XY-women). And what about XX-men?
In general people are assigned a gender based on their phenotype, whether they have male or female sex organs. But some people are born with both male and female organs. How to classify them?
Then we have people who don’t identify with their biological gender, but with the other gender. The people are usually called transgenders. Some of them seek to undergo a sex-change operation, the transsexuals. Many jurisdiction allow these people to change their legal gender as well.
We propose a four gender classification rather than a three gender one. Beside female and male, we propose to introduce intersex and neutral as legal gender. Intersex are those people with both female and male characteristics, while neutral are those who lack both male and female characteristics.
People are assigned any of these four genders at birth. However, people who feel that their legal gender does not fit with them, will be allowed to request the civil registry to change their legal gender. No operation would be required to have one’s legal gender changed.
Another idea would be to abolish official gender registration at all. But we believe that will cause certain awkward situations. Whatever system will be chosen, we believe in equal rights for everyone regardless of one’s gender.
Lagrangian Republican Association 
I think a 4-gender classification system might work…I imagine society would have to make a lot of adaptations to it though, right down to language. Even today, the question of how to refer to transgendered or non-binary people raises problems; I’ve seen some who prefer the indeterminate “they” as pronouns, while others who’ve made entirely new ones, like “xir” or “zwie” and things like that. I imagine it would be useful to have separate pronouns for intersexed people and non-sexed (neuter) people.
>>I imagine it would be useful to have separate pronouns for intersexed people and non-sexed (neuter) people.
I agree. The use of “they” is, however, problematic as it is the plural third person pronoun. and I have yet to see that “xir” or “zwie” will be widely adopted. Personally I don’t have a problem with either of them.
In the draft version of the Mordan language (which is based on Ido), we have il(u) for men, el(u) for women, ol(u) for neutral and lu for persons in general.