As Justice Thomas said, it's not the actual ruling that's so abhorrent (I want all such laws off the books) it's the reason -- privacy. Santorum will be proven right in short order. Some freak in Utah is going to argue that he can have sex with his 2 daughters and 2 nieces (all of age, of course) and the incest and polygamy laws cannot enter his bedroom.
He'll probably lose -- the first time (some penumbras are more equal than others) -- but it won't be long before some appeals court will have to acknowledge the right to privacy there.
The government has a plethora of compelling reasons to keep sex between a father and daughter or nieces very illegal just a it does for bigamy and other crimes. To argue differently is simply hysteria.
This decision was a good one. The government has absolutely no business in the private bedroom of consenting adults, period.