A pair of artists (and at one time, friends of mine) Named Norithics and Chaytel both approached the same idea.

They left this site, mostly due to petty politics, (Involving this particular topic in no small part) but their arguments and analysis were very compelling.
They migrated to a site called Inkbunny. I didn't follow; more out of laziness than any real cause I assure you.

Those two dudes are positively brilliant and hilarious, you may want to hunt them down.

Personally, I can only speak from my own experience and from the time I have spent caring for children.
I have always understood the concept that females are attractive, and I have also been attracted to breasts in a sexual content for as long as I remember. But the sheer mechanics and concepts behind sexual intercourse greatly disturbed me until puberty. (It's kinda creepy, now that I think about it; it was literally an overnight change)

From the time spent both rearing siblings and watching for the young of others, I have seen somewhat similar ideals; where children are very aware of the differences between genders, but the mechanics of intimacy are considered alien (Oftentimes children would tease each other with accusation of kissing, in order to provoke a reaction of disgust and defense).

As an adult, I find myself having adopted a philosophy where human sexual nature is considered a discipline and an art more than anything else; both in terms of understanding desires as well as how one behaves with a prospective partner.
Of course, there is an entirely different discipline at work behind finding and understanding a prospective partner in the fist place, but you asked about sexuality, not attraction. ;P
As a discipline, I have felt that sexuality is a concept that must be approached in stages. Thus, children shouldn't consider any particular notion in depth until they have gained understanding of the precepts which lead up to it. I guess it can be a bit of a peculiar convenience, but I have often seen that children will be curious about what particular notions of sexuality that are appropriate to their age all on their own, unless specifically given reason to do otherwise.

I like to think that this approach was taught to me by my mother, and that directly facing and analyzing myself and my instincts has been a healthy practice. But that's a judgement entirely based on personal views, obviously.

I believe a lot of the concept may be entirely physical and hormonal. I've some claims that children are hitting puberty at younger and younger ages. I honestly don't have any real ideas as to how that would affect emotional development.