[CASTE:FEMALE]
The gender tag lets it know how breeding works.
[FEMALE]
[MULTIPLE_LITTER_RARE]
To add beards, put square brackets around the following:
BODY_DETAIL_PLAN:FACIAL_HAIR_TISSUE_LAYERS
[CASTE:MALE]
[MALE]
[SET_BP_GROUP:BY_TYPE:LOWERBODY][BP_ADD_TYPE:GELDABLE]
[BODY_DETAIL_PLAN:FACIAL_HAIR_TISSUE_LAYERS]
This is what sex looks like for Dwarves right now. More genders should be as easy as copying and pasting and renaming. Tokens for determining creature descriptions, frequency and such can be found here: http://dwarffortresswiki.org/index.php/DF2014:Creature_token
Modding guide: http://dwarffortresswiki.org/index.php/DF2014:Modding_guide
Oh so I could add like
[CASTE: TRANSSEXUAL FEMALE]
[MALE]
or [CASTE: GENDERFLUID]
[MALE]
Then a genderfluid with female, a transsexual male caste, etc etc?
DF's tags and raw files are incredibly flexible, but they are
not a Turing-complete language. Raw modding is limited to what tags exist in the game, which is why there was no way to manipulate sexual orientation until this release with its new ORIENTATION tags. The tags' parameters are a bit confusing, but for any civilized creature the default is to have a 75% change or being interested in marrying the opposite sex, a 20% of being interested in taking the opposite sex as a lover (but not marrying) and a 5% chance of being uninterested in the opposite gender. There is a completely separate die-roll for the same sex: 75% uninterested, 20% lover and 5% marrying. So having a Dwarf end up straight, bi, gay or asexual is already achievable.
The reason I can summarize the whole thing in three sentences is because sex has no game impact outside of reproduction. In short, they are
all genderfluid since they're all shifting across all possible genders continuously (which is one).
The [MALE] tag makes a caste
* capable of fathering children, the [FEMALE] tag makes a caste biologically capable of bearing children, and the [GELDABLE] tag in a bodypart allows a creature to be sterilized at a Farmer's Workshop. When the game does get around to socially constructed genders, they will likely be defined at the entity (civilization) level and obey whatever randomization exists for every other civilization-level characteristic like ethics, values, noble positions, etc.
*Caste doesn't have the expected meaning in DF. Anything that requires the tags to be different for some members of the species than others defines a caste. Many monsters have one sexless caste (called DEFAULT), most animals have two (MALE and FEMALE), bees have three (WORKER, DRONE and QUEEN), and ants have four (WORKER, SOLDIER, DRONE and QUEEN). Note that WORKERs and SOLDIERs have no sex tags, and in the intelligent version (ANTMEN) they never form relationships. Probably with ants and bees in mind, a sexless caste in a create that has sexes uses the feminine pronouns.