"Two sexes," they say. "Any others are unnatural"

MUSHROOMS: but we have 17,000 sexes. Whoops sorry just discovered another one. Make that 17,001.

"The female of the species," they proclaim, "is the one that carries the fetus to term. Males can't get pregnant. It's unnatural."

SEAHORSES: but our males do.

"It's so simple," they declare, a little nervously, "If you were born with a penis, you're a male. Boom."

SPOTTED HYENA: but our females have penises.

"If you were born with a vaginal canal, you're a girl," they scream. "If you weren't, you aren't. End of story."

TURTLES: but our males are. and they can even breathe through them.

"Biological sex can't be changed," they shriek, self-righteously."That's just science".

CLOWNFISH: but we do it all the time

"You can pretend to be a gender you weren't born as, they gasp, assuring themselves that surely this time they will emerge victorious, "animals can't just up and change their chromosomes. That's common sense."

BUTTERFLIES AND MOTHS: but we can. explosive chromosomal macroevolution is literally a part of our lifecycle.

"But chromosomes determine biologi-"

BEARDED DRAGONS: ours are determined by the weather

"But the Bible says male and female he made th-"

BLUEHEADED WRASSE: all of us are born female

"But you can't just wake up one day and deci-"

GROUPER (MALE): we literally do.

"But gender isn't flui-"

GROUPER (MALE): sometimes we change back

"But you can't just pick what gender you're going to be on any given day!" They shriek, "It doesn't work that way!!"

GROUPER (FEMALE): ahem

@Lana Makes me wonder. Could it be that species get ‘trapped’ into a local optimum regarding gender and sex, possibly because ‘brains’ consume so much energy, everything else that ‘works for now’ stops evolving or is simply disabled? (The BBC article mentioned in this thread shows that we still have the genetic capabilities to change sex at a later stage)
@Dave_von_S @Lana It's not completely unlikely. E.g., some subspecies of groupers "lost" the ability to sex change which is linked to bigger territories with less likelihood of "not enough males/females" and allowed for bigger testes in the males.