Prop. 8 is a serious threat to marriage. Not just to those families whose marriages it will annul, but to all marriages.
Proponents of Prop. 8 are right: Marriage is at risk. They are wrong about why. Marriage is at risk because people consider it optional for living together and starting a family. Denying marriage to tens of thousands of couples will make this worse. Same-sex couples will have to consider marriage optional, because they have no choice. And, if they don't need a marriage to form a family, why would anyone else need one?
This is what Prop. 8 really says: "Marriage is optional; you don't need to have one."