And what legal principles underlie those laws? If it is religious or tradition, that is gone with an Olsen-favorable ruling. If it is proceation, that is also gone, since the Olsen case seeks to invalidate procreation as a legal precedent for "marriage". Strip those away and I see nothing to prevent an easy challenge to both incest laws and incestuous matrimony.
I am opposed to gay marriage.
However, Olson is going to argue that Prop 8 violates the Equal Protection Clause and this Court will but that argument and overturn Prop 8.