The intent of the rule is to prevent BEA users from posting photos of people who appear to be under the age of 18, so by "similar sites", we generally mean any web-community or photo-album site where one can obtain photos of people who appear to be minors.
Added by Pal: Just to expand a wee bit on this... since we can't list EVERY possible site that could be problematic per the Owner's wishes, we have to settle for "W, M and similar" ...if a user has a doubt about whether subjects in a photo are possibly underag3, or about the source of the photo, then that user ought to err on the side of caution and NOT post it here. It's simply not worth the risk to the site.
Added by gonZo: Nimrod, as far as I can tell, the five Stretcherr images on the prohibited site weren't exclusive to that site, and can be found in his BEA Member Area gallery, his free gallery, and this forum.