"...So sex isn't realistically any different, only culturally."

I agree. But we have to look at where we are now, culturally, and ask ourselves: if I get into a sexual relationship with this kid, is she going to incur some sort of harm from that relationship? Unless some truly exceptional circumstances are present, I think the answer is an overwhelming yes, and I no longer believe it can always entirely stem from iatrogenesis years down the road.

If Lily consents, and then I put my hand in her panties and start masturbating her, and she feels good, I may not be directly hurting her, but the social implications of what I'm doing certainly are.