While not universally taboo, there is at least some associated with even non genetically related people. Cousins through marriage or step-siblings for instance. Also it occurs with friends--"I couldn't date him, he's like a brother to me."

On another note, why would someone producing offspring with higher risk of retardation because of incest be different than a lot of other things. People that know that their offspring might have down syndrome are generally not ostracized for producing such offspring, for example.

I cannot help but to have a negative emotion towards insest in my gut however; I do not know if it is because of society's influence or some intrinsic value.