Sure, the first time someone goes to watch a video promising "Hot lesbians working out" only to find an old man discussing the different stages of aerobic cellular respiration, they will chuckle a bit before continuing on with their search. But once a point is reached where the porn sites are so flooded with fake videos that users have a hard time find what they want, it will get incredibly frustrating.

And lets face it, despite the fact that 4chan is pretty random, the bulk of its content is pornographic, and the bulk of it's users spend a lot of time searching for, and watching porn online. If this idea has ever come up on the boards, it's pretty likely that it didn't gain much of a following because for those users that watch porn online, it would essentially be the equivalent of shooting themselves in the foot.

Along with that, I'd think that porn sites would actually monitor the content submitted more carefully than YouTube would. With Youtube, content is either removed because it involves copyright infringement (whuch is the biggest thing they look for), or if it's offensive. Since they can't be sued for offensive content, they more or less leave finding it to users reporting it. The only problem is that offensive doesn't just mean nudity; it can mean violence, animal abuse, excessive profanity, some type of criminal activity, or simply that a user disagrees with the ideas of the video. Basically the porn thing worked on youtube because the deleting of content is focused on copyright infringement, and there are so many different videos reported for different reasons that it's hard to sort through them all and find the ones that are truly needed to be removed. With porn sites, content is removed if having it on the server is illegal and could get the owners arrested or cost them a ton of business. Anything posted on a respectable, legitimate porn site, is screened to ensure that it doesn't contain anything such as CP or beastiality that could get them in some serious trouble. In this process it's fairly likely that anything irrelevant would also be screened out as well since that sort of thing would decrease traffic to their site.