The truth of the matter is, it's really all just a matter of symantics (spelling?). Part of that, is a great deal to do with western society, and how sexually repressive we are. Particulerly in the more religious areas. We are taught, to some degree that sex is dirty, sexual thoughts are something to be kept to oneself, and so forth and so on. The reaction to Janet Jackson's exposure of her breast is a prime example of how screwed up our society is.
That asside there is also a lot of semantics when it comes to art. Michalangilo's "David" isn't considered porn, despite the fact we have a naked male. Neither is other pieces of art which prominantly display nudity. In writing of mainstreme books, i've read scenes which are quiet graphic in sexual content by famous authors, and yet their books are not considered porn either. Hell, i once picked up a romance book to leaf through it and see what was actually in one. For the most part it seemed like a pretty normal story until i hit the page involving the women stroking the guys erection in graphic detail after snaking her hand into his pants. Yet, this was being sold in a drugstore and there were no warnings on it. A ten year old could have probably bought this book.
Ultimately the difference between porn, erotica, inbetween and otherwise is how many mothers and bible thumpers gather to bitch about it.
Oh, and to answer the question, i would call material which is created for a sexual interest to be porn. However, porn can mean a lot of things and i don;t think we should get hung up on the fact that it could be considered porn, save for legal reasons.
Something you have overlooked: Child's clothing catalogs are not designed to be jacked off to. Tickling clips are, and that makes makes them porn.
