Namely, it says that childhood is when you have your innocence, and adulthood is when you don't.
I'd just like to make clear that my post characterizes YOUR attitude as in-line with this statement, not mine. Yes, I realize you disagreed with it.
The point of using "when you have sex you are no longer a child" is to put the dividing line between child and adult at physical sexual maturity, and to allow the player to choose when that point comes. It has nothing to do with innocence or economics (granted, I can't speak for anyone else who includes it in a playbook they write).
Also, Joe, I should point out that you're the guy who complained that the Battlebabe's special was "boring" because of it's "negative" nature, that it didn't produce any rules-results. And yet, like any other rule, it's a designer's statement of their understanding of systems (how things work), and incentivizes/disincentivizes certain play behaviours. You say you want to "explore" the issue -- are you sure you're not really asking for rules that immediately confirm your own viewpoint(s) while being unwilling to consider this rule on it's own terms?
I find there's already a lot of nuance there, even though all of my knowledge contradicts this statement:
I know for absolute positive that ten and eleven year old people can engage sexually without being corrupted; without becoming adults; without losing any essential innocence.
Specifically, I'm wary of using personal experiences to essentialize or universalize. Whether or not something is possible and whether that possibility is even statistically relevant or not are two different matters.
At the same time, I realize that's in some sense what we do with role-playing games. We're exploring and sharing narratives, not realities, and those narratives are built out of personal experiences (and fears, desires, expectations, and fantasies, of course). In choosing to use that particular wording, I AM actually saying it's inherently broken and damaging, the same way violence is - while at the same time catering to the designer's desire to include sex moves, and the desires of most (and I would argue it IS most) players' desire not to deal with child sex in their games. But I don't agree that it shuts down discussion or exploration completely.