There are no exceptions to it if you make it the definition. Which is what the comment of BuyMyBitcoins says was taught to them.

Yep, fair enough, if it's a strict definition. You could potentially then have weird edge cases in the other direction, though; if the same sex chromosome layout in related species X and Y leads to different sexes due to the size of the gametes (especially if the size of the gametes was pretty similar to start with), that seems very odd/confusing.

It looks like there are cases where the size of the gametes are identical. But when this happens they don’t refer the organism’s sex as “male/female” and instead use “+/-“. https://en.m.wikipedia.org/wiki/Isogamy

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.

Here's a fun edge case: https://en.wikipedia.org/wiki/Indotyphlops_braminus#Reproduc...

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.

A closely-related species using the same or similar chromosomes, where male/female exist, is likely where it came to be that the observed individuals of these snakes have all been determined "female", even if no "male" has been observed.