You people are also forgetting that most other species have a much more complex gene pool. There's more genetic variation in a large bee hive than in the entire human race. I know from personal observations that an animal with the intelligence of a housecat will mate with their own children if there are no other fertile males around, and cats are (compared to some other things in the animal kingdom) pretty smart. It's basically like saying that a cat would be stupid for jumping out of a 15 foot tree because if a human did that they'd probably walk away with a broken limb. After several generations of incest the cats will start to come out with more problems, but in MOST animals once or twice is the exact same as mating with someone you're not related to.