Obviously genetics is the main reason why incest is frowned upon to say the least. And this is actually a pretty great reason from an evolutionary point of view. Species that maintain some genetic diversity are much better off, which is why systematic inbreeding is a pretty bad idea, and things like the (albeit unproven) Westermarck effect are highly beneficial. I think that the aversion society has to consensual incest is a huge overreaction on an individual basis, but a great idea on a larger scale. I also think cultural imprinting is what causes most people to think it's "gross". We've been taught this ever since we were young and maybe the Westermarck effect or similar has made us relatively "repulsed" by our own family, so it makes sense. (By the way, I have pretty much the same reaction: rationally I think it's okay and won't oppose it, but it does feel kind of icky.)
But let's be clear: genetics pretty much is off the table. At least in the way most people seem to be considering it here. Most sex doesn't lead to procreation, and even if it does, the probability for genetic defects isn't increased by that much in one generation. To be honest, I don't really think we should be making any legislation based on this. For that you need more than just genetics, unless you want to prohibit many, many more at-risk couples from having children. Should every couple take a mandatory genetics compatibility test? And what will be the increased risk percentage at which we tell them they can't have children? And what about other factors that could decrease the quality of life for potential offspring? I'm pretty sure it sucks if your father smokes or your mother is a total asshole...
In light of this, I think there are definitely situations in which it would be okay for siblings to marry and have children together. Even if I don't ignore genetics completely. However, I do wonder how often that is really the case. Incest seems to open up a lot of opportunities for dysfunctional relationships, especially if different generations are involved. I would not think it was okay if an uncle or a mother groomed their cousin/child into loving them in such a way that they'll have sex with them when they turn 18 and could be considered consensual adults. This is obviously not the only way in which things could go wrong. I think that the kinds of relationships (involving loyalty, respect, etc.) that exist in a family increase the likelihood of a romantic relationship being dysfunctional. Whether we should base any laws on that, I don´t know, but I´m certainly not comfortable saying that any consensual incestuous relationship is aces with me. I´d want to look at it case-by-case.