You don't have malware. Many adult sites trigger "pop-unders" on just about every link you click. It's in their site code, and pop-up blockers don't work. You can disable javascript, but usually that will make the site you want to use stop working too. MacKeeper seems popular, just don't click any links or install anything. Glimmer blocker will block the site, but it doesn't stop an empty window from opening.
There's not much you can do about it besides closing a window now and then. There's a price you pay for free porn.