porn can appear violent to a child. my first glimpse of sex came from an accidental viewing of a rape scene from a soap opera, and after i saw that (i was 11), i had terrible views on sex and was horrified by it.
that first sentence, plus my own first viewing of sexual intercourse, justifies keeping porn away from young eyes. now, i'm sure that if i hadn't been conditioned to think that sex was an act of gentle intimacy, it wouldn't have been so bad. but there you have it.