What it is that's "none of our business"

Prominent libertarians have argued with indignation that the doings of the FLDS are nobody's business, and that they are victims of religious persecution and prejudice. Here is an example of what the FLDS is about. Now, if consenting adults want to create plural marriages, they have that right, and I even believe it should be protected by the Free Excercise Clause, something the courts have rejected. But if adult men think God wants them to sex up little girls, that's where they violate individual rights and go beyond the freedoms libertarians ought to defend.