It's a weird line, that I think guys have a hard time following, because there's not really an equivalent for them. Guys and girls really are two very different genders. For girls there really is this whole world that exists for them that doesn't for guys.
Like...think of it this way. And note that I'm not providing any moral judgment here. For guys, there's this whole world that girls don't really experience, where they're taught, typically from a very young age, that they might have to be violent in order to protect others. You can say that's biological, or cultural, it doesn't matter - the point is that it exists, and it's not something that girls really experience.
For girls, there's the whole idea of looking pretty, that their bodies are a somewhat sensuous work of art, and yes, they get introduced to this from a young age. It's not even a modern thing, either - even something as simple as, say, a ballet outfit, that little girls might get into, is much more revealing than anything you'd expect a boy to wear, and ballet - even at that young age! Is an art form that's all about the artistic curve and line of women's bodies. And an inherent part of this is yes, it's somewhat sensual, so even while little girls who get into it might not be thinking about it in any sexual way - just like little boys playing with toy guns might not be thinking about what it's actually like to die on a battlefield - the fact is sexuality is a part of it.
And I think we used to have a healthier idea about this all - that yeah, there might be shades of sexuality in what children participate in, but they're participating in a generally non-sexual context - before the internet came around. Because the internet came around and like the anon said before, it exposed the fact that there were in fact a bunch of adults viewing this in a very sexual way. In the past they'd just be some weirdo in their home, barely ever exposed to society. Now everyone can see them.