I'm sorry to be condescending, but all you who attribute human norms to DF dwarves are boring. Also you are incredibly lacking of imagination.
So, who says dwarves are bi-gendered? Who says they are monogendered? Who says they are sexually dimorphic? Well, the RANDOM NUMBER GENERATOR DOES!
It would be trivial to have these factors be randomly generated. Dwarves, or any race, could have any number of genders. Each gender could diverge have systemically from the other genders, and these differences could be along arbitrary attributes.
Thus foobarale dwarves are generally faster and more crafty than the more lumbering but intrinsically funny and administrative barbazale dwarves. Both these genders are greatly attracted to dwarves of barfoole gender, who have an particular affinity for cooking and tend to be great with weapons, but also be moody and depress easily. They form mate-relationships in three or four, and barfoole dwarves spawn offspring when simultaneously impregnated by one (or more) foobarle and barbazales.
Honestly (while I certainly don't think dwarves need human gender dimorphism -- some versions give female dwarves
beards, and having dwarf genders have essentially the same strength isn't really a problem compared to that), randomness to that extent can be a bad thing.
Particularly in an ascii game like Dwarf Fortress, it's very important that players be able to visualize things from relatively sparse cues. This doesn't mean that they have to exactly reflect reality, but going for completely strange genders and so forth that have no relation to anything in human experience is, in my opinion, not a good idea (except for really weird creatures like elder horrors and demons, which are
supposed to be strange and impossible to visualize.)
I recall that Armok 1 had really freaky random creature generation for everything and, basically, I don't think it added to the game. Having a world players can relate to is important... there's room for really freaky creatures on the fringes (or in a mod or whatever), but something called a 'dwarf' should match at least some fantasy archtypes, and having more than two genders doesn't really fit into that.
...also, there are humans in the game. Should we represent gender dimorphism for humans? I don't think it's terribly important, but I don't think it would be a horrible disaster, either. (One possibility: Represent gender dimorphism accurately for 'typical' humans, but declare that all adventurers are by definition 'exceptional', so it doesn't apply to them -- in other words, adventurers aren't a random sample; any female adventurer is going to be freakishly strong, which is not uncommon for female adventurers in fantasy or legend.)