For me the question is "What does a warning accomplish?" to which my answer is "cover for plausible deniability" ("we didn't know kids were looking at all these naked folks. Besides we put up a warning so they should have known their actions would make ours morally questionable"). I'm not sure they provide much in the way of defense against kids seeing porn, just a legal way to assert one's lack of culpability in providing it to them. As someone hoping to reduce the pressure on sexuality in the legal realm, this would serve as a one severing of that connection, though not the most important or most harmless (which are not necessarily the same things).