I think DART should have an "other" option in the gender category, in case someone doesn't identify as female or male (e.g. nonbinary people). This is pretty much the status quo on DDO.
Even for those of you who believe that gender is equivalent to biological sex, there's a whole community of intersex people who don't fit within the binary.
In any case, even if gender is actually a binary (which I don't think it is), there is no harm in letting people who don't think it is classify themselves as not being female or male.