It could be very useful to add a feature on the taxon edit page (or any other place) that allowed to specify how the sex on that species work; like monoicous (bisexual), dioicus (unisexual), asexual, variable (for some higher taxa and probably some specific cases) and unspecified (for the ones that have not been specified at the time).
Then the sex annotation could be linked to it; just as the “Life Stage” Annotation on insects is linked to a higher taxa; and so the annotation “Sex” could appear or not in the Annotation Box depending on the option marked on the specific taxon. I think that this should not be made all over iNat directly, but been slowly added, and probably starting to add this to the higher taxa,the ones that common curators cannot change, like marking Plants (Kingdom Plantae) as variable; Vertebrates (Subphylum Vertebrata) as dioicus (unisexual) etc.
Possibly it would be also useful to add an intersexual option to de Sex Annotation, because although they are very unfrequent, they still exist.
I probably missed some specifications that could be made for this feature, so feel free to point them out!