Grownup sexual issues in the sense of acquainting one's genitalia with someone else's body parts are (mostly) theoretical for (not too precocious) children! Issues of one's sex are decidedly NOT. From a very, very young age - maybe for boys it doesn't become non-theoretical until middle school, but I'd laugh at the idea that girls aren't hyperconscious of gender expectations after the age of about five. MOR!Hermione is constantly comparing her relationship with Harry to "Romances" she has read, expecting herself to fill such a role under constant societal encouragement and reinforcement of how girls just act that way and melt in a variety of creative manners whenever they so much as think momentarily of love. That's something she never ever would have been exposed to and acting upon if she hadn't needed to visit McGonagall in December.