Why should there be an objective boundary?
Etymology: Greek pornographos, adjective, writing about prostitutes,
from pornE (prostitute) + graphein (to write); akin to
Greek pernanai (to sell), poros (journey)
Date: circa 1864
1 : the depiction of erotic behavior (as in pictures or writing)
intended to cause sexual excitement
2 : material (as books or a photograph) that depicts erotic
behavior and is intended to cause sexual excitement
3 : the depiction of acts in a sensational manner so as to arouse a
quick intense emotional reaction <the pornography of violence>
noun plural but singular or plural in construction
Etymology: New Latin, from Greek erOtika, neuter plural of erOtikos
1 : literary or artistic works having an erotic theme or quality
2 : depictions of things erotic
Variant(s): also erot·i·cal /-ti-k&l/
Etymology: Greek erOtikos, from erOt-, erOs
1 : of, devoted to, or tending to arouse sexual love or desire
2 : strongly marked or affected by sexual desire
Of course, if you want a more realistic definition than the abstract
interpretations of linguists, go ask someone in the erotic industry
what they consider to be the difference between what they do and porn.
That will give you a better idea of why the difference is important
to a real person, and hence why it appears in the language.