This thread has turned into complicated but really important territory and I just want to say (again) that it seems to me we are not all talking about the same thing.
Some of us are talking about sexual boundaries as an idea thsat's clear in our own minds, to which I would say, fine - that's good. But the point is that young children don't have clear ideas about either sex or boundaries yet. Can a child's play be "sexual" if the child has no sexual purpose in mind and has no idea about sex yet? I remember one evening I came to check on my son in the bath, and there he was, lying in the bubbles with an erection, pretending it was a space rocket taking off. Was that sexual? Not for him.
There's also the problem that some of us are talking about what should occur, while others are talking about what does occur. When I point out that kids are curious about their bodies, including their sexual parts, I'm not commenting on whether that's a good thing or not. I'm just saying this is a well-attested fact, going right back to the Kinsey reports (that is, almost 60 years ago).
I also think we have to acknowledge that our own take on the sexuality of children is full of big triggers for us. Some of this stuff can feel REALLY dangerous at an immediate personal level, but if one guy says his experience is such-and-such, let's not gang up on him for just stating what he saw or remembers.
Nobody living can ever stop me
As I go walking my freedom highway.
Nobody living can make me turn back:
This land was made for you and me. (Woody Guthrie)