Dwarf Fortress has announced that they are working on adding trans dwarves, and they'd have added then already of he hadn't overcomplicated his thinking.
.... last time when he added gay dwarves he rewrote a large chuck of the personality system so that gay, lesbian, ace and bi dwarves would all arise naturally from the system, so that kinda makes sense.
If a given dwarf rolls "disinterested" for the same sex, and "commitment-possible" for the opposite sex, then that dwarf is strictly heterosexual, and is willing to become married. If the dwarf rolls "lover-possible" for each sex, then the dwarf is bisexual, but will never marry. And so on.