Not that I have any familiarity with the industry, but I think that a lot of porn outlets actually pioneered this method of affiliate and referral content linking.

I think you're right, it is an inherent risk in the fact that these are supposed to be trusted networks of partner sites sharing content with one another and so mainstream sites you wouldn't expect deliver malware and viruses through this vector as well. I just think this segment of the infographic folds legitimate sites with the NSFW ones.

I'll give you another example somewhere between porn sites and respectable mainstream sites...

You ever get sidetracked by those "Trending on the Web" sidebars that deliver external content that is usually sensational, tabloid-style stories? Things like a red circle around a portion of a frame from a movie like Harry Potter, or a story like, "10 things girls don't know they're doing wrong in bed"... Those are the same basic methodology of delivering content that we're talking about here - and those will quickly get you into networks of affiliates that are rife with malware. Those don't really show up as an individual category here, either... but I think it is because they're all included in the two categories you define.