The gut reaction to the mention of incest more often than not is negative,because the first thing that comes to mind is not consensual sex between siblings, it`s adult-child sex.
Difficult to think that this anything to do with "consensual" .
The stipulation that it "does no harm" is a bit weak too.I can`t recall ever seeing any press about a well adjusted adult saying that an incestuous relationship was a good thing in their life . However, I have seen way too much crap like this,
http://en.wikipedia.org/wiki/Fritzl_case.
Don`t know about the rest of the world, but in the U.S. one of the most used
defense tactics in criminal cases is that " so and so was sexually abused by a family member".
Conceptually , in terms of a human`s right to do as they choose, incest could have some basis of support , but in practice it does not show any positive results.