That's probably the most depressing thing about this. A taboo taken for granted since time immemorial is no longer taken for granted. One now has to come up with arguments against incest. And given what the courts are like throughout the so-called developed world, there's no guarantee of winning the debate. Sanity might win in the short term, but the handwriting is on the wall.
The inbreeding argument won't work anymore, or soon won't. Based on risk/benefit assessment, it's purely utilitarian; and not everybody can be expected to believe that the relevant calculations of utility outweigh arguments from alleged individual rights. Leviticus and sacred texts from other religions prohibit incest without using the inbreeding argument. The real argument, it seems to me, is from the psychological integrity of the family. But in our age, fewer and fewer people know or care what that means.