I can't even say that I necessarily have a problem with that, but what gets me is the idea that incest is exactly like slash (starting at such simple things as newsletters sorting Matt/Mohinder with Nathan/Peter), and that, consequently, any negative reaction to incest is basically the same as homophobia.

I wish I could say people aren't that stupid, but I know some of them are, and I understand your reaction to this equation. Luckily I don't have had any relevant rl experience, but I still feel put out whenever I encounter it, too. Sadly, discussion is basically futile precisely because of what you describe - it always ends with: "But if you say you're squicked by my kink, you're a homophobe/racist/impediment to freedom of speech!"

Mind you, it's of course possible to avoid reading something that squicks you because the pairings are labelled. (For example, when I see something credited with Noah/Claire, I definitely won't read it because the very idea grosses me out. So does Sylar/Claire, for that matter, but that's another squick.) But I realize some fandoms make it harder to avoid pairings that rub you the wrong way than others - you can spend years in Alias fandom without coming across incest pairings (I know I did; I still would assume there aren't any if Yahtzee hadn't told me otherwise), whereas this really is impossible with Heroes. (Or SPN, I guess, but then the only Supernatural fic I've read were crossovers with other fandoms.)