The Peanut: Hey mommy, I have this great idea. When we grow up, me and The Bean will get married.
Me: Grrshfflpgt!? Umm, err, no sweetie, you can't do that.
The Peanut: Why not?
Me: Umm, because you're not allowed to married your sister. Its a rule, brothers and sisters can't get married.
The Peanut: But mommy, you told me that people who love each other should be allowed to get married. And me and The Bean love each other, so we want to get married and live together forever.
Me: [gack. I did tell him that. And it was the right thing to say. Because its true. Except for this one, tiny, little exception. Think, woman, you're a parent, you have to be able to deal with these things] Ok, I did tell you that. Except, here's the thing. If you marry someone else, and you have kids, and the Bean marries another someone else, and she has kids, then your kids and her kids will be cousins. And you know how awesome cousins are, right? (Oh yeah, I love my cousins) But if you marry each other, there won't be any cousins. See?
The Peanut: Ok, I get it now.
So, in my house, the reason why incest is bad is that your kids won't have any cousins to play with. That works.