My rules give some explanation. (Everyone reads my rules, right? XD Maybe I should link to them everytime I mark “RULES VIOLATION" or “RULES WARNING”.)
Anyway, my first encounter with incest in furry (not exactly pornographic) was in “Better Days,” a webcomic by furry artist Jay Naylor. The chapter I linked is where it happens (or is heavily implied, anyway). That was probably my main turn-off to incest in pornography.
More generally, sexual reproduction between closely-related individuals is disadvantageous in natural selection, because it does not create as much genetic diversity. So there is a biological reason for avoiding incest, which could lead (justifiably, in my opinion) to the cultural avoidance of it.
However, I think incest has more to do with the relationship between the individuals. Adoptive siblings are not closely related biologically, but I would still consider sex between them incest. Family relationships (I think) are asexual (going back to the biological advantage), and since the relationship between adoptive children and other members of a family is about the same, I would still consider it incest (and thus wrong to some degree).
Now the Celestia-Luna exception. You might be referring to this post, in which I mention the exception; I also mention it in my rules. In the rules, I point to a clopfic, "Tipsy” by Anonymous Pegasus on FIMFiction, whose handling of the incest desensitizes it a bit to the cultural taboo.
More generally, I make the exception because, frankly, Celestia and Luna have a pretty flimsy claim for being “sisters.” All we have for that is the first episode, but there’s no history of their birth, their parents, or their childhood. (I doubt that they’re even sisters in the traditional biological sense.) Like I said, I think incest is about the relationship, so the fact that there isn’t much of this relationship between Celestia and Luna makes it seem less like incest than if the “sister” claim is taken on its face.
You know, I read that comic myself some time ago, so I know exactly the scene you’re talking about. I honestly don’t have a clue why that inherently makes it a turn-off.
For the record, I definitely don’t approve of long-term incestuous “relationships”, because things get entirely too complex on an emotional level. And I’ve always felt that efforts should be made that inbreeding never happens, for obvious reasons.
Personally, I find that reasoning for Celestia and Luna incest to be… odd. Flimsy logic even. But I guess if it works for you, more power to you.
I guess I struggle to understand why people are against incest porn.
To me, it’s simply a matter of having a good time with people you trust; as all sexual encounters should be. Why is that so bad?
I actually really loved the incest plot in better days and thought it was handled very well.
Then again, I like Jay Naylor’s work in general, so a lot of people would think my opinion is invalid.
In re needs-more-plot and sirfredrickoftrottingham on “Better Days”:
I have to admit that I wasn’t clear on why this scene in “Better Days” was a turn-off (EDIT: punctuation). To be honest, I have to admit that I don’t really know myself. That comic just made my head spin throughout, so I think it just made the “shock” of incest worse.
I guess I like Naylor’s work, but I tend to avoid it because a lot of times the creator gets pretty political (both within and without his fictional works). (Even this whole chapter in “Better Days” seemed to be just a political tract.)