Would differentiate between 'acceptable' and 'legal.' Not familar with Youtube (old pc can't view flash-based content by and large,) but have seen the 'ptnn' type content. I'm not a fan. Since 3d cgi sorta porn got good (1st generation stuff was unviewably bad,) real-people porn but for the occasional vid (like 70s era sorta porn of the legal variety) doesn't show up in my collection. In highschool I was, back then I had everything...But I experienced the same sorta guilt post-climax others often do with such material that over time, and as 3d animated alternatives got better looking, migrated completely.
For me, viewing any sort of ptnn content, even if technically legal, presents ethical issues. The models involved are being used for content with no real value except as technically-legal child erotica. Whereas hand-drawn hentai/manga, or computer-generated 3d type content involves no one except the artist and is thus no more objectionable than stick figures from the 'internet porn simulator' from that South Park episode.
May look very realistic, but since it isn't using real people is both legal, and guilt-free. For now. Suppose emphasizing the guilt-free aspect will ensure the anti-porn types lobby it away eventually, but for right now federal US law has it legal.
(8) “child pornography” means any visual depiction, including any photograph, film, video, picture, or computer or computer-generated image or picture, whether made or produced by electronic, mechanical, or other means, of sexually explicit conduct, where—
(A) the production of such visual depiction involves the use of a minor engaging in sexually explicit conduct;
(B) such visual depiction is a digital image, computer image, or computer-generated image that is, or is indistinguishable from, that of a minor engaging in sexually explicit conduct; or
(C) such visual depiction has been created, adapted, or modified to appear that an identifiable minor is engaging in sexually explicit conduct.
(9) “identifiable minor”—
(A) means a person—
(I) who was a minor at the time the visual depiction was created, adapted, or modified; or
(II) whose image as a minor was used in creating, adapting, or modifying the visual depiction; and
(ii) who is recognizable as an actual person by the person’s face, likeness, or other distinguishing characteristic, such as a unique birthmark or other recognizable feature; and
(B) shall not be construed to require proof of the actual identity of the identifiable minor.
(10) “graphic”, when used with respect to a depiction of sexually explicit conduct, means that a viewer can observe any part of the genitals or pubic area of any depicted person or animal during any part of the time that the sexually explicit conduct is being depicted; and
(11) the term “indistinguishable” used with respect to a depiction, means virtually indistinguishable, in that the depiction is such that an ordinary person viewing the depiction would conclude that the depiction is of an actual minor engaged in sexually explicit conduct. This definition does not apply to depictions that are drawings, cartoons, sculptures, or paintings depicting minors or adults. http://www.law.cornell.edu/uscode/text/18/2256
Above is the classification anime et al. falls under (11). 3d cgi is a bit of grey area and think some places have already made it illegal even though real minors aren't involved by virtue of it being 'too good.'
So ptnn stills or video may be legal, but it's not also acceptable. And a relevant quote comes to mind:
"Never forget that everything Hitler did in Germany was legal." Dr. Martin Luther King Jr.
"I'd rather be hated for who I am, than loved for who I pretended to be." - Me.