According to the definition posted above, the cultural expressions of biological sex are only one dimension of gender, and you’re ignoring the other aspects. I’ll accept fish don’t have culture, though I bet someone more knowledgeable than I could argue that point. However, let’s look at social behavior via a vis courtship rituals. Like birds, some fish develop pretty incredible displays for getting it on. If a fish which has changed its biological sex then changes it’s behavior during courtship, that would seem, to me, to indicate a different expression of biological sex independent of genetics (i.e. gender). Unless there is a genealogical basis for courtship displays, which I don’t believe to be the case.
Would you care to make a case for Lady in the Water? I don’t recall a huge amount of that movie, but what’s there is not positive. Admittedly, his gall in writing a character who is destined to save the world through his art, and then casting himself in said role, left a pretty sour taste in my mouth, but I shouldn’t let that affect the other aspects of the film.