why is incest wrong?

[minionofmidas]

The risk for first-cousin offspring is only marginally higher than for people generally - comparable to that for the offspring of mothers over (what was the exact figure? Somewhere in the 30s. Pretty low in the 30s actually, IIRC.) For siblings, it is of course also lower than for parent-child relationships, but higher than for cousins.

Wouldn't the rate of risk be higher for siblings than for parent-children, considering siblings have the same genetic material, and a parent only has half of the child's?

Uh, unless they're identical twins, siblings don't have the same genetic material. They have anywhere on a fairly random scale from no shared genetic material at all to being identical in all but one chromosome (minus the spontaneous mutations). Obviously the ends of the scale are less likely than the middle. Parent and child, though, will share half their genetic material.

Obviously this assumes that these siblings were not themselves descended from incestuous parents. The risks are sort of cumulative. European High Nobility is a small set of families that have been intermarrying for centuries. Small wonder there haven't been any non-deformed people born to them for a while. [/slight exaggeration]
Sexual experimentation among siblings soon after puberty is surprisingly common, especially in large and/or relatively isolated families, and usually ends pretty quickly as well. (I know a story about it from my own family. But I don't think the people in question know that I know.) In and of itself, I wouldn't think that there's anything wrong with people as do that, as long as there isn't a huge age difference. Now, if it goes on for years... something may be amiss.
There's also the scientifically demonstratable bit about how siblings who weren't raised as siblings tend to be sexually attracted to each other to an unusual degree.

Weren't two of your grandparents twins?
[/quote]More like my grandparents' grandparents, but apart from that, yes.

But this is the other side of my family.

[minionofmidas]

What cultures allowed incest, anyway?  (Honest question; I don't know.)  Did any of them not fall into the amoral warrior society mold?  You haven't done much to prove it's not a matter of correlation more than anything.  That's what I was getting at.

Eh. The easiest division of society (well, the part of society of the opposite gender) into "people I may furgle" and "people I may not furgle" relies on considering one set of cousins "honorary siblings" and the other as fair game, usually with completely different words for them. Cross-Cousin Marriage, it's called. (In Latin, for example, the early system got lost somewhere around the classical period, but before that they had what anthropologists call an Omaha Kinship System, after the native american language for which it was first analysed. It is indeed the most common classification of relatives around the world, although others exist.)
My father's daughter is my sister.
My father's brother's daughter is my sister.
My father's father's brother's son's daughter is my sister. etc. The same goes for my mother's sister's daughter - indeed, if the pattern is unbroken (but in reality, it frequently is), it's the same thing.

But my mother's brother's daughter is my future wife.

[Willy Woz]

Because the stupid religious right tells you it is.

[Beet]

Because of the risk of inbreeding, which societies have generally developed a cultural defense mechanism against by considering incest deeply taboo.