So there's something in the meta tags or somewhere that makes at least SOME web filters believe this site is pure porn.

Maybe the "Art" tag. We know how dangerous that can be.  I know that at least one military command's proxy blocked this site. Many corporate proxy servers ban gambling, gaming, sports, news, social networking etc. sites not just porn since they are not work related. I assume that the porn, gambling etc. companies have their own lists. Larry Flynt probably doesn't want his employees on-handed surfing on his dime either.

Try going to a railroad art site and see if it blocks you.

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken