What's wrong with incest?

It seems we've been socially conditioned to view it as wrong and sick. But, why is that? I once had a crush on one of my cousins. What's so wrong and gross about that? Some people even consider it wrong and sick to be attracted to a step-sister or step-brother, who wouldn't even be blood-related. People confuse me.

And some of the same people that believe it's so wrong and sick also believe that the entire human race is descended from one couple -- Adam and Eve. If they believe that, then they have to believe that every couple on the face of the planet is incestual, including themselves if they're in a relationship, because all of humankind would be one big family. Incest would be completely normal, and unavoidable unless no one ever reproduced, in which case we would have died out a long time ago.