Something went wrong in these peoples' lives or in the wiring of their brains. For some reason, they fundamentally don't understand the difference between children and adults. Thus, they project their adult interests and desires on these children. It's a fairly common phenomenon with other, more traditional, pedophiles.

All of this obsession with self-identity is so adolescent, and it boggles to the mind to see so many grown adults obsessing over ridiculous stuff, like what gender they feel inside.