While I'm naturally disgusted by incest, logically I can't see a problem with it if it's between consenting adults... unless they have kids, then their actions will affect more than just them.
However, there are no laws to stop other people likely to pass on generic deformities from reproducing and survival of the fittest is long gone. Not saying this shit is right, but maybe it should be treated the same as a couple with other genetic defects who chose to breed.
There's also the argument - used by some Royals (and Roman Gods?) I believe - that inbreeding strengthens the gene pool by passing on more "good" than "bad" genes.
It's still fucked up though, whatever way you look at it.