When is art porn or vice versa?

Finally, I've begun a new painting. I have two I am still working on and a third begging for attention with an idea, but no composition. This new one took on a life of it's own from the start, but wasn't planned that way. Are the others supplying too much pressure that I needed a good dose of freedom? Am I rambling in oil now? Is this a doodle while listening to a lecture? I don't know, but it looks good and has me pretty excited. It has a great glow so far and the shapes are intriguing, but it beckons another question, when has a shape gone too far?
One can argue that a shape never hurt anyone and therefore cannot be as much the culprit as the viewer construing added meaning. But one can argue that no meaning can come before the shape. The swastika is a shape. It clearly represents evil things, but is the shape itself bad? If found in the future buried in a forest, long after the human race eliminated itself, it would only be a clue and a design, not the Nazi movement, yes? Of course I would have to concede that once other clues of our race were discovered then yes, the shape of a swastika could be redefined as representing something evil.
Now, in my paintings I am not trying to defend swastikas nor do I include any, but I am wondering how far is too far when it comes to shapes. In particular, when does art become porn? Is the outline of a proportionally large penis porn? Are labial folds porn? Is an erotic pose porn? It is a valid question, but it is also one I do not want to define per se. It's a question that would be terribly interesting to answer because it has so many implications and would be a very interesting conversation over a few beers, but not one I want defined exactly. In fact, it would have been THE perfect conversation back in college when I was single. It begins with intelligent insight and the sharing of knowledge on art and history and humanity, but it would serve the dual purpose of negotiating the imaginary boundary line between each others views on sexuality. You would know right away how far the date could go and which base was art and whether you could steal home AND whether porn will be involved later.
I digress, where is the 'line' drawn? In my latest painting, started just last night, clearly some well formed breasts are prominent. For the record, I am a butt man, but(t) this doesn't really come into play when I paint. My painting is usually about something that, to me, feels beyond sexual boundary and societal convention. I paint ideas and emotion and if a nice pair make there way on to the canvas than it's nice scenery during the process. That brings up my true contention, I believe the boundary between art and porn is mostly limited to intent anyway. What does the author (or digital pimp) intend? What is meant by the image in question and for whom is it meant? It's a good question and one might be persuaded that the point of intent is enough to leave it at that. I say, "Buuuuuutt..."
I say, "buutt," because intent can be so gray. First of all, we may never know the true intent behind an image. Even if we do, society may down the line decide that the intent behind a particular image isn't all that bad. Public opinion changes. In the past it was at times black and at times white and still at others both black and white, but when considered over time it is gray. In fact, I would say more and more things become gray as more and more information is catalogued, made readily available online, and easily compared or conjured with the click of a mouse. That is a completely different conversation however.
Secondly, there is artistic porn and pornographic art and graphic sexuality with an artsy touch or even 'good porn'. What of the line being drawn as a genre of it's own? The gray just got bigger. It is much like oil painting there, gray is actually one of the most important and most difficult to master treatments. It is the gray that makes the color. And maybe that's the thing, it's the gray area that makes the art?
Maybe a demonstrative question that could shed some light is, what is my intent in this new painting? I intend to reach the viewer, engaging the imagination, and starting a visual conversation. It just so happens that it requires boobs to do it in this particular dialogue. If I meant to arouse you, that very image would change before our eyes. If it comes off too pornographic though, it limits the audience and salability. Does that limit the conversation then? When does the line cross the line?