Because it is an opinion. [/b][/quote]When two relatives (eg brother and sister) have a child, chances are higher of having a not normally functioning/deformed child. Not very fun for the child, and for the parents. When they do want to have a child, it can be seen as "weird" and not very thoughtful, IMO. Whether it's "wrong" is another discussion.

I presume that the fact we think incest is weird comes from the human biology. Clearly we just know, from birth on, that incest has it's risks. What you seemed to point out as a cultural thing, is just our biologic alarm that is cultivated and generally accepted.