How many of your sexual experiences have lead to offspring? how many of anyone's has?
I think jumping straight to the 'what if' of inbred offspring is side stepping the actual issue. Two consenting people who are capable of understanding what they are doing, should absolutely be preserved the right to have any manner of sexual interactions with each other. This is their private business and a basic human right I think.
The majority of sexual activity does not result in offspring so this should not even be brought into the debate. Also, even in the case of inbred kids, they are not always born fucked up. They are more likely to be totally fine.
As far as the 'taboo' niche's of sexuality go, incest is pretty fucking mainstream and has been common forever.
It is not for everyone sure, and it grosses most of us out. But would not this disgust be equatable to homophobia? Think of all the rhetoric "it is not natural!" seems identical.
Let me guess, you have a very attractive cousin?