There are many miserable things about the history of humanity, but children's sexuality was not claimed to be a major problem until quite recently.

That's precisely the point. Concern about ill effects of early sexual experiences has become a problem because of appropriately rising expectations. (You and I will also agree as far as saying that it's not as big a problem as some segments of society believe.)

It's appropriate that expectations rise as conditions improve. When there is serious famine, a family might be excused for selling off a child to keep everyone else from starving -- it certainly happened. A man giving food for the opportunity to f*ck a young girl (against her will) but not taking her away forever could be considered a better deal. That's an extreme case, but I think it "scales well" with less extreme cases too.