"Pornography", to my mind, is something for which the degree of sexual titillation is the only thing that counts. There is no attempt at artistic quality. (I am aware that some would not agree with this definition.)

i don't exactly disagree, because that definition sort of works for me too, if just as a starting point for discussion -- but it has the same disadvantage as all the other ones:  it's still personal.  unless i know somebody put an image out there solely with intent to sexually arouse people, with no artistic idea in mind, i can't rightly pontificate on how that's "clearly porn", because i am not a mindreader.  with some things it seems pretty obvious -- they're artistically so bad that i'm tempted to deny them the "art" label right away.  but really, defining art by quality doesn't work either.  some imagery i might want to call porn because it's in self-identified porn magazines -- hustler, penthouse, playboy ... woops.  some of that stuff actually looks like people cared about the expression of the model beyond the spread legs, and lots of care is taken with lighting, with setting a mood.  when does that become art?  and this is all a whole lot harder with writing, because i find writing more difficult to separate from art than imagery.  so i end up, once again, saying "to me this image says nothing at all artistically, it just displays some 'fuck me' pose, therefore i classify it as porn", but i fail at finding a definition that works across the board.

the distinction between erotica and porn is also not per se personally important to me -- i don't actually sort my images with tags like "porn" versus "erotica".  sometimes an image turns my crank, and that image might well have been created as art, and part of what attracts me is the artistic value -- but i am not really looking at it with the art in mind whilst my crank is being turned, ya know?  other times even a really flagrant image without apparent artistic pretensions does nothing for me.  actually, most times self-identified porn does nothing for me, and i require some artistic values to even get into it.  that really smudges that porn/erotica line.

The distinction must be made, otherwise one concludes that there are subjects that cannot be treated by the artist, which to me is an abhorent conclusion.

that would be a very bad thing, i agree.  but i don't actually see why we need distinctions to prevent it, and, being as that distinction between porn and erotica is attempted by lots of people, i don't see it working all that well.  if you create distinctions, people will immediately start arguing about where the line should be drawn.  i think instead that anything and everything should be fair game for the artist, no subject should be taboo.  i rather not draw a line at all that's supposed to be meaningful for everyone.  i am quite comfortable with drawing my own personal lines, and letting everyone else do the same.