I've paid to be a member of a racing forum. Why? Because I liked what the forum was about and I knew it took money to keep it up and going. If a girl has a site and she does content that I enjoy, why wouldn't I join her site every now & again to support her efforts?
But yeah, like hissexslave said, if you're spending your last dime on a porn site (or anything else for that matter), or if it's affecting your outside life, that's the sign that you have a problem. The free or pay part of it has nothing to do with it, IMO.