I think that in order to claim that one in five American males is a sexual abuser of children, the definition of "abuse" would have to include "sexual talk aimed at teenage girls who are demonstrating sexual availability" by "males" who are just enough older than the girls to trip over a statute.
We can't have a culture of rampant fornication and also have a culture that (legally) treats postpubescent youth as if they were 3-year-olds, without plunging into absurdity.
I guess no one really knows. It’s possible that incest could explain it, but 1 in 10 seems more reasonable to me.