First off, ignore PaperLuigi. Some people buy into this whole thing about 4chan being some sort of secret society. It's not, and the people who think it is probably learned everything they think they know about the site from Encyclopaedia Dramatica. Also, 4chan != /b/.
With that out of the way, I'd guess your problem is that your current IP was used by the proxy service someone else tried to use to evade a ban. Are you by chance on a satellite Internet connection? If so, you'll see this a lot if you try to browse 4chan. Some IPs won't be banned, others will, usually for varying reasons.
Note that you won't be able to appeal this ban, even though it is by all means wrongful. 4chan isn't a big deal, so don't worry about it. And while it's not entirely a porn site, it does have some boards devoted to the stuff.
