Brave New World Update--If Gay Marriage is OK, Why not Incest?

Michael Medved and Dennis Prager, among others, have cautioned that the same arguments used in favor of gay marriage would suggest that incestuous marriage be permitted as well. The problem with these slippery slope arguments is that they have a way of becoming the next "progressive" agenda item. Here is a news story from Germany about a brother and sister who just want to be free to love one another. Interestingly enough, the story does not mention legalizing marriage, just incest, presumably because the institution of marriage already is passe' in Germany even among unrelated heterosexual couples.