It’s Not Porn, It’s Art: While not a legal distinction – for obvious reasons – the important distinction between the socially acceptable (and indeed regarded as something of an art form) erotica and the socially unacceptable pornography is that the former requires (a) appealing characters, (b) some semblance of a plot, and perhaps most importantly (c) that a jolly good time be had by all involved.
(Yes, this disqualifies about 95% of the Earth material about which the question might be asked.)
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