lol, I just spent quite a while reading this topic. I like how he flat-out denies that incest between brother and sister causes birth defects. There's a fellow that got married to his cousin, and both their kids have physical and/or mental defects (both due to their parents being related and their parents being idiots themselves).
Who brought nazis up in the first place by the way? The discussion ended right thar, .
If you continually marry your cousins, you can be more prone to disease if it's in the family. See the Egyptian pharaohs if you want to know how. But back in 19th century England it was perfectly acceptable to marry your cousin, like Charles Darwin did. So it generally depends on where you're from.
But it is almost always unacceptable to marry your brother, sister, mother, father or anyone in your immediate family.
I think the rule of thumb is three (generations?lineages?) of separation and then you are pretty much safe.
It's only really dangerous if you start to continuously do it within the family, and if you have a prevailing recessive genetic defect. The chances that you get to recessive traits go up because you are more likely to inherent both recessive genes from the parents, rather than at least one dominant.