I can imagine a future generation where gender fluidity becomes
  a side-effect of human life expansion [physicists make
  everything spheres, so a genderless human would be a logical
  result].

  and at around the age of 4 years old, children are asked by
  their caretakers to chose their gender. Little 9xj2.997a.alpha
  picks up the annoyed Thomas the Train and points it at
  caretakers face.
  [1]no
  "But honey, school is starting soon. You have to choose."

  9xj2.997a.alpha picks up toy train again.

  etc.

References

  Visible links
  1. http://icopiedyou.com/wp-content/uploads/2015/08/no.jpg