... and worms and sponges, and possibly plant-creatures, as well.
I guess this could be easy-ish to implement in theory - you'd basically just consider them females that simply need any other member of their species on the map to trigger pregnancy (or just search for two of that creature type), provided they are in the same caste, although it might get... very strange if you apply this to a caste amidst other castes that have male or female tags.
Which is presumably part of the reason that Toady has not done this already (aside from presumably not caring about the mating habits of worms in general)... although making [HERMAPHRODITIC] creature-level and overriding of other gender tokens might solve the problem in a simple solution, as that prevents caste-level redefinitions so that if any member of the species is a hermaphrodite, all creatures of the species are hermaphrodites.
Otherwise, there might be strange implications for if someone decides to make a third civ species (I.E. dwarf) caste of hermaphrodites, and how that would play out in terms of the marriage mechanics.
There is also the issue of self-fertilization. Some creatures shield themselves from self-fertilization, but some creatures, and I believe this applies mostly to plants, are capable of fertilizing themselves, and have no particular defenses against that.
If we are going to talk about hermaphrodites being added and self-fertilization, as well, though, there's also the matter of
parthenogenesis, which is where females' eggs functionally self-fertilize. (Which is technically not the same as a hermaphrodite fertilizing their own eggs, it's purely the eggs own self-fertilization.) Parthenogenesis is extremely rare among vertebrates, but common among invertebrates such as insects, as well as, again, some plants.
As the wikipedia article shows, there's actually a species of lizard that is all-female, and parthenogenesis-based. They technically aren't clones, as there is genetic variation based upon the way in which the genetics of eggs are already pseudo-randomized in meiosis. (Unless the eggs were produced in mitosis, in which case, they are clones. WHEE! Complication!)
A queen bee will also lay eggs that self-fertilize if she has access to no drones to sexually fertilize her.
These will be male, due to the way bees use chromosomes in a different manner than our standard "XY" system of chromosomes.
Isn't it funny how a little bit of facts and knowledge can turn even the simplest of things complicated? Nature is so much Fun.