i think that even you will admit that the above is more the exception than the rule.  your above example from skin deep also lacks nudity, which i believe - in some form - is an essential component  of pornography.
its a pretty safe bet that most often nudity + sexual acts = pornography.

nothing in this world can be defined by an unwavering, straightforward definition. there are always exceptions.
that said, i should refine my personal 'opinion' though:
erotic art is meant to arouse, and pornography is erotic art that depicts explicit sexual acts. i left that out above.

while i think erotic art (in advertising, marketing, media, etc) definitely has its place in our everyday world, pornography is better left behind closed doors. renderosity is a community trying to reach the broadest public - family - audience. if anything, as they market to a wider audience the gallery and marketplace restrictions will get tighter, rather than laxer.

bobasaur - thats an interesting point. if one were to go to renderotica, for example, and select 50 images at random from  the  various softcore/hardcore galleries there...
then assign each image a ranking of 1-10, with 1 being innocent artistic nudity, 5 being eroticism, and 10 being shameless pornography.
then give each of the same images a 1-10 ranking on quality and artistic merit, with 1 being the lowest quality and 10 being the highest, i am willing to bet that you would notice an inverse relationship between explicitness and quality.
of course this is a massive generalisation, but based on my past experiences id be willing to bet its pretty accurate. there are of course exceptions -- ive  seen some very talented people there put out work that is pure pornography and in total contrast to what i just said. but overall in my experience there, here at rosity and in other galleries the above relationship is more or less accurate.

the conclusions you draw from it, of course, are highly debateable.