'Sexual expression which is indecent but not obscene is protected by the First Amendment,' Sable Communications of Cal., Inc. v. FCC, 492 U. S. 115, 126 (1989), and except when protecting children from exposure to indecent material, see FCC v. Pacifica Foundation, 438 U. S. 726 (1978), the First Amendment has never been read to allow the government to rove around imposing general standards of decency, see, e. g., Reno v. American Civil Liberties Union, 521 U. S. 844 (1997) (striking down on its face a statute that regulated 'indecency' on the Internet). Because 'the normal definition of 'indecent' ... refers to nonconformance with accepted standards of morality,' FCC v. Pacifica Foundation, supra, at 740, restrictions turning on decency, especially those couched in terms of 'general standards of decency,' are quintessentially viewpoint based: they require discrimination on the basis of conformity with mainstream mores.