Does it make sense to make incest illegal, or should that be left up to the participants themselves?

Politically speaking, are there any rational reasons for making incest illegal?

Would you like a sexy dependent manipulative psycho but loyal sister/gf?

>You are not to have sexual intercourse with your sister, either your father’s daughter or your mother’s, whether born at home or born elsewhere. You are not to have sex with her.

- Leviticus 18:9