Bro, how sheltered was Haruki before the change? Because even if you weren't the kind of boy who would watch R-rated movies without permission or sneakily look up erotica out of sheer curiosity, you almost certainly knew such boys among your friends and classmates. And I mean, like, even before you're out of elementary school.
I get what the author is going for here -- the implication is that, being a TS girl, Haruki didn't have aunts or other older female family members warning her about the world that someone raised female from childhood would have had. It'd make sense for her to have considerably less defenses, but not zero.