The above reply was "right out of bed" answer. Thinking about it more, I was trained in part by analytic philosophers, so I'm attempting a conceptual analysis, trying to carve language at the joints to figure out what it is about sex/pornography that we need to protect children from.

I'm not arguing that sex is exactly like every type of adult activity--if that were the case, paying taxes would be a lot more fun. But I don't think "emotional signification" can power the conceptual analysis needed; both tongueless kissing and fighting with friends are extremely emotionally significant activities, but they are quite often shown in G-rated movies.