In 1957, in Roth v. United States, obscenity was defined as 'a speech about sex that is utterly without redeeming social importance'.1 Two more criteria were added in the sixties, 'patently offensive' and 'appeal to prurient interest in sex' (which sets 'good' pornography apart as it may have artistic value or arouse non prurient desires). 
But as the Supreme Court pointed out, the line separating what is obscene (thus illegal) from what is pornographic (and as such under First Amendment protection) is often dim and uncertain.

In 1973, Miller v. California modified one more time the definition of obscenity. Instead of 'utterly without redeeming social importance', the new criterion became 'lacked seriously literary, artistic, political or scientific value', with all the ambiguities regarding the appreciation of 'value' in artistic works. Another major change was that from then on, 'contemporary community standards' were used to decide whether a given material was 'patently offensive' or appealed to 'prurient interest in sex'. 

This implied that depending on the State or the city, the same material could be judged illegal or not. 
This legal frame was strengthened by the establishment of another category of speech, indecency, which, though not obscene, can all the same be restricted by law. It applies to 'patently offensive depictions or descriptions of sexual or excretory activities or organs'. These various definitions, because of their looseness, leave room for subjectivity and led to numerous convictions for obscenity and indecency by local courts. Such was the legal apparatus on which the PMRC relied in its efforts against rock music.

  Censoring sexuality in popular music.

It was not the first time popular music was being attacked. Until the late eighties, obscenity prosecutions against music had been virtually inexistant but it does not mean that music had not been the target of censorship. The issue is obviously too large to be dealt with here satisfactorily, but a few facts may prove useful to delineate the context in which the PMRC developed. From their early days, jazz and blues had often been labelled 'the devil's music' or 'jungle music' and as Count Basie recalls, they were often described in terms of 'orgies' (Heins, p.80). 

African-American genres frightened because they expressed every aspect of human nature, including sexuality; the rampant discrimination against African-American culture was thus made legitimate and respectable since censoring these musics was presented as a crusade for decency. While black music had been directed at the black market, no one had really objected; it is only when the white youth began to be attracted that the attacks really began.