I recall the multi-million dollar space probe that failed because one group of engineers had used the metric system and another the English system of measurements.  At least both of those systems are based on replicable physical reality and one or the other would have sufficed for the utilitarian purpose of measurement.  Such objectivity simply does not apply to questions like whether Mapplethorpe's work is art or pornography.

It's fine to believe in some holy grail of artistic objectivity but I don't think it exists.  Entire atristic movements are persona no arta to the academy one day and hailed as great the next.  Eurocentric authorities completely dismissed African and other non European works that are now acknowledged.  What changed, the art or cultural atitudes?  There may well be some validity to artistic evaluation if based on something empirical like perceptual psychology.  You can determine that more people like red paintings than blue paintings perhaps though I still don't think one can honestly define one group as more "artistic" than the other. 

Beyond that, it's pretty much fodder for intellectual masturbation amongst the elites.  The vast gulf between what people really like and what the critics and academicians tell them they should like is the cause of many people saying 'I don't understand it so it must be art.'  Of course, the 'authorities' have every  right to establish their criteria and declare who is in or out but their pronouncements are hardly relevant to the vast majority of real people in the real world.  The fact is that most people asked to explain why they like something are not going to be able to expound in terms that would delight a critic, they just like it period.  Perhaps for them, art bypasses the rational brain and gors straight to the emotional and esthetic - which I would think is the real idea of art to begin with.

As for art/erotica vs. porn, if one can view some explicit images like Mapplethorpe's or Indian temple friezes or whatever as art rather than pornography than a definition based on how graphic an image is becomes unworkable.  Do we say that an image of as man inserting a whip into his anus is art if shot as a profile but porn if the distended sphincter is shown?  And would it be impossible for the latter to be shown in an artistic manner?

Objective, functional definitions may have utility.  A flaccid penis is OK but an erect one isn't; a dildo touching a vagina is fine but the slightest degree of penetration is taboo... Those are things one can work with, things that enable unambiguous communication of information, fairly enforceable rules etc.  Anything else may make for entertaining conversation or rancorous debate but nothing really useful. 

On the contrary, subjective criteria lead to chaos and are always subject to a variety of political, religious and other forces that guarantee that result.  I can't legally rent an 'XXX' rated movie in my county but I can drive a few miles and rent as many as I want.  OTOH, I can go to the local convenience store and buy a magazine with images that depict all the explicti acts that appear in the banned movies.  We used to be able to buy them at a local newstand but they were forced to close because they sold magazines that depicted the same acts between two men that were acceptable if performed between men and women or even two women.  It may be fun to debate smut vs. genteel stimulation or whatever the alternatives are supposed to be but the reality, at least in the US. today is that both are likely to be tarred with the same brush. and the finer points of theory and sensibility have no influence whatsoever.

"Democracy is a pathetic belief in the collective wisdom of individual ignorance." - H. L. Mencken