> The idea that second graders have "open" sexual identities

“Transgender” is not a sexual identity, it is a relationship of gender identity to gender socially ascribed at birth, usually on the basis of the appearance of external genitalia (but possibly on the basis of genetics where that has been previously tested.)

And people usually have an open gender identity by second grade.

(Second grade also isn't particularly early for children to have an established sexual orientation, though it's a bit earlier than the median age for that.)