I say there is no such line at all. How can there be a thin line separating anything in a creative field?? A thin line is an engineering feat, which has distinct differentiators to be able to separate them from each other. There are a million lines by many eminent people trying to explain how art, erotica are different from Porn. Many go on to add that Porn is about a Sexual Act, and is intended for arousal, gratification, whereas Fine Art Erotica are more of subtle seduction. What each one is effectively saying is what they think about it, because am sure no sane person will ever try to dictate how a creation is interpreted.
I feel, what a person makes of a situation is entirely based on his upbringing, his surrounding culture, his education, and his exposure to life. A school boy with raging harmones will consume all work as porn. Where as a person with exposure to museums, education, art related surroundings, is more likely to seek art even in a hard core pornographic creation ?
So I feel, we must stop trying to label creations, instead the concentration must be on consumption, soaking it in for oneself, and not instant interpretation for others. Let each person decide for himself/ herself of what the creation means to them.