It's been years, and I'm still waiting for the coherent logical explanation of why incest (and also polyamory/polygamy) is necessarily wrong. I think people just have a gut ick reaction. I've heard people say "inherently abusive", but I've never got a full explanation as to why or how it is. If someone claimed that such relationships have statically lower rates of working out well, that sounds very likely. I would easily believe that. But the government having to sign off on the assertion that your relationship is a good idea is really invasive. For most other topics, there's a difference between weird/messy/ill-advised, and criminal.

I do think there's a decent chance the plural marriage thing might become a public debate at some point, but I doubt incest will, just because it's such a fringe issue. The number of people who are in love with close relatives is so low that I really doubt they'll ever have the necessary mass to make people pay attention.