WHEN THE BBC invited me onto one of its talk shows recently to talk about the day's hot topic -- legalizing adult incest -- I thought of Rick Santorum. Back in 2003, as the Supreme Court was preparing to rule in Lawrence v. Texas, a case challenging the constitutionality of laws criminalizing homosexual sodomy, then-Senator Santorum caught holy hell for warning out that if the law were struck down, there would be no avoiding the slippery slope. "If the Supreme Court says you have the right to consensual sex within your home," he told a reporter, "then you have the...