Wrong. All it takes is a browser fingerprint. If they can associate that with a name, whether or not you're logged into the porn site — e.g., through sites that you are logged into — they can identify your browsing session on the former.
It seems like denying access to my plugins (and maybe non-standard fonts) would solve this. Why does a website need to know my plugins in the day and age of Flash being deprecated?
"Web browsers leave an essentially unique footprint every time you visit a web page, even in Incognito mode (and even without supercookies). This is well established; many web tools such as Panopticlick will confirm that you give a website lots of information about your computer every time you visit."