Why should there be an objective boundary?
pornography
noun
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>
erotica
noun plural but singular or plural in construction
Etymology: New Latin, from Greek erOtika, neuter plural of erOtikos
Date: 1854
1 : literary or artistic works having an erotic theme or quality
2 : depictions of things erotic
erotic
adjective
Pronunciation: i-'rä-tik
Variant(s): also erot·i·cal /-ti-k&l/
Etymology: Greek erOtikos, from erOt-, erOs
Date: 1651
1 : of, devoted to, or tending to arouse sexual love or desire
<erotic art>
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.
....Roy