if it makes you feel any better:
they’re referencing the many species which are born as one sex but can change sexes later in life. many fish species are like this, where they’re all male or all female and then one fish in an area will morph into the other sex. although tbf I’m not aware of any shark species which have sexual metamorphosis traits
and this is a spoiler but it’s based on the mythos of the island with the tribes being descendants of sharks.
There are many species on land or in water which are unisex, some can even reproduce by themselves in real life too , I mean on our earth. I dunno you have read all this but this is author’s fantasy mythology and also there is a sentence meaning “bc they can’t find mate easily in deep water” kinda (that’s true in real life too) … I think that explains what you want to know.

Till the “if you take on a women’s role during your first sexual experience then you evolve into a women.” WHAT??? I’m sorry but I don’t like the implications of that at all.