Nudists take nudity - something that, in a textile society, is often associated with sex - and interject it into an otherwise totally non-sexual lifestyle. Now, stepping back from the textile perspective, it's not altogether illogical to consider the unclothed human body separate from the act of sex. One would have a harder time, however, making a similar argument for erections, which are a physiological process designed to prepare the male organ for sexual penetration (of the female). Nevertheless, there is some little room for that argument, as erections may be stimulated psychologically or even with incidental contact, without the presence of, or even necessarily the intent to engage in, any explicit sexual conduct.
[description: series of portraits of a naked man playing volleyball outdoors with an erection]
I would not spend too much time arguing that the selective exhibition of such cases of erection-absent-sex (with "sex", I suppose - by a broad definition - potentially including such things as manual stimulation - e.g., with the hands or other body parts - or actions such as thrusting of the hips) would not carry with it some level of erotic intent on the part of the producer/distributor, or that it would not primarily be viewed in such a fashion by its consumers. However, it would seem to strike an ironic parallel with nudists' attempts to "desexualize" nudity: by featuring erections in everyday situations, without making any overtures toward the organ's sexual function.
Is there such a thing as a "non-sexual erection"? And even if we must conclude that there is not, could there not still be individuals with an interest in erections depicted in non-sexual contexts? Some might say, "what's the point? You're showing an erection. Most people would consider that sexual. You've already crossed that line, so you might as well go all in." But I think that the novelty of an erection appearing outside of a sexual context could very well have a unique appeal to a subset of the population. After all, in the realm of pornography, "softcore" may be considered by many to be a synonym for "watered-down", yet it remains a popular subgenre for a great many people who can appreciate the subtle art of suggestion over a more direct, in-your-face approach. I, personally, while tending to lean more toward softcore, maintain that both approaches are interesting, and worthwhile to explore.
If anything, though, I think this demonstrates that erections themselves (independent of what's being done with them) should not necessarily be considered "sexually explicit". How is a non-sexual scene with an erection any more shocking or alarming than it would be if the penis it featured were flaccid? What is it about the shape and size and turgidity of a penis that agitates people? Is the stiffness of a penis uniquely offensive in a way that a view of the organ itself, in its restful state, is not? Or is it because it is liable to invite questions, or direct one's thoughts toward the "immoral" subject of sex? (Although, in a textile society, one could say the same thing about nudity in general; the fact that nudists have divested themselves of this assumption is only evidence that the same could perhaps be accomplished in a more erection-tolerant society).
I feel (and I could be wrong - this is just my personal impression) that a great proportion of the perceived offensiveness of pornography is in its vulgarity - the closeup on body organs, the liberal depiction of bodily fluids, and the great diversity in the distinction individuals make between what is appealing, sexually, and what is disgusting (as well as the variability in a single individual's opinion depending on whether or not he is currently aroused - a character flaw the Marquis de Sade once cautioned against). None of this would seem to me to hinge primarily on whether or not a penis is hard or soft. I maintain that if you have the maturity to see a penis in its flaccid state, then there is no reason you should not be able to handle the sight of it erect.
