That seems like a reasonable idea to me. I started a thread discussing a similar topic a few months back: Life stage annotations for “Recent Evidence” observations (e.g. leafmines, galls, juvenile feathers).
“cannot be determined” could be useful for animals with no sexual dimorphism, tracks & signs, etc.
I’d also like a “none” for Plant Phenology so plants which aren’t flowering/fruiting/budding can be annotated.
Much of the time it doesn’t really matter but if I’m trying to verify that I’ve annotated all my flowering plants Plant Phenology then being able to put something against every observation makes it easier to check. Same with sex & life stage for animals.