Anyway, certain things are taboo pretty much across all cultures. Incest is one of those examples (here I'm specifically thinking between siblings). But - why exactly is it a taboo?

Obviously a child born to people so closely related by blood would be at an extreme genetic disadvantage, possibly leading to many physical/mental problems. So are we just naturally wired to stay away from incest for this reason? Is this pretty much the only reason, then?

What about people who don't want to have children... Why then would their relationship still be taboo?

What about siblings who are not related by blood (one or both adopted)? Why is that relationship taboo?

Disclaimer: I'm absolutely against incest of any kind (that whole cousins being allowed to marry in certain cultures pisses me off ).. but I'm just trying to understand the reasons behind this pretty much universally unaccepted act.