Going a step further I found this: >Depending on the group, different mating types are often referred to by numbers, letters, or simply "+" and "−" instead of "male" and "female", which refer to "sexes" or differences in size between gametes. https://en.m.wikipedia.org/wiki/Mating_type

So it does seem to be a strict definition and anything outside of the strict definition is given an alternate identification scheme.

All individuals of this snake are female (presumably 'female' here is defined relative to other snakes) and reproduce parthenogenetically (sic?). By your definition, these wouldn't have a sex at all, presumably.