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.
"Your browser fingerprint appears to be unique among the 5,053,325 tested so far."
"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."