
Haven't read the entire thread - so my thoughts may be superfluous in adding...
Major arguments, apart from, 'it's sick' appear to be in asserting genetic issues, such as increasing mutation and deformity.
While part of this is an evolutionary issue, and that being debatable - depending on what you think about that theory

I tend to agree, though am not entirely convinced.
MUTATION AND DEFORMITY.
Importantly, incest itself doesn't automatically lead to deformity, and would lead to even less mutation. Such instances of deformity are a result of a heriditary genetic disposition, and issues relating to recessive and dominant genes. With this being the case, it simply highlights an existing familial condition - and if there is none, none will show.
Essentially, if genetic variety, and change is the goal, then incest is 'bad', but if you have a genetic line that you wanted to maintain, then incest would be the way to go.
The proof, as such, is best seen in the intermixing of races - where 'incest' amonst a single race is extremely unlikely to lead to offspring with features semi-unique to another.
INCEST AND ABUSE
And this is where I would assert incest to be inherantly abusive. That it is impossible for an incestuous relationship to be anything but, where one party will have developed a position of authority/power over the other. This would be most obvious in a parent child relationship.
On the other hand, this might appear not to be an issue between close siblings

which is where my predjudice comes in

gross.