I simply see no problem with incest among consenting adults. It doesn't hurt me, you, or anyone else, at all. The law is there to prevent harm, and nothing else. When it's used to enforce a moral code, as in a "victimless crime", it's almost guaranteed to be unconstitutional. Homosexuals marrying doesn't hurt anyone, just as incestuous marriage doesn't hurt anyone; they should be legal. I really don't see the point of marriage between relatives, since it doesn't grant them many rights that they don't already have; this is the big difference between it and SSM, since the Gays don't have these rights as the law stands.
The law isn't there to enforce morality, or what any group deems "common sense"; I don't understand the point of drag-racing, sky-diving, or jig-saw-puzzling, but there's no valid reason to make them illegal. The actions of others don't need to "help" me to be legal, they just need to not "hurt" me; if drag-racing, sky-diving, or jig-saw-puzzling "hurt" me, then I'd have a claim for why it causes harm and needs to be banned. (Like lawn darts) Until you can prove that SSM, Incest, or incestuous Marriage (Given that they are all consenting adults), has harmed you, it's unconstitutional to ban them. Our opinions on any of those things are not relevant to a discussion of their legality. For the most part, I have no opinion on marriage or sexual relations between others, of any configuration. It's none of our business.
Personally, I dislike most sports, reality tv shows, and most magazines; but they aren't hurting me. I've never claimed that they should be illegal, even though I truly dislike them.