To those saying "ignoring genetics is handicapping the discussion" - the point of this thread is to talk about the morality, which is not necessarily linked to, shall we say, consequences. The other way to frame it would be that we're only discussing cases which don't end up in procreation.
That out the way, my stance is basically meh. I think there's a danger of incest being based in unhealthy relationships caused by a lack of normal social interaction, but that's not an argument against incest itself. More against, well, a lack of normal social interaction!
I guess I'm saying that if two (or more, I guess!) people who are related, mentally healthy and socially well-adjusted choose to get it on, there's nothing wrong with that. Like many others though, I will state that personally, ick.