Freedom of speech is established by the First Amendment: "Congress shall pass no law...abridging the freedom of speech." Free speech is regarded by many Americans as one of their most important rights, but, there are efforts to regulate, restrict, or prohibit many types of speech — often with popular public support. These efforts are commonly motivated by religious objections to sex, blasphemy
, etc. People approve of free speech, but only when they approve of the content. When speech involves political views they dislike or sexual material that offends them, the principle of freedom of speech disappears.
Is pornography good or bad for society? You'll find liberals on both sides of this question, and quite a few who don't regard it as necessarily good or bad. Almost all conservatives, however, do say that pornography is harmful and most attempts to censor sexually explicit material come from the politically and religiously conservative. This is arguably a violation of free speech and expression guarantees, but perhaps even worse it's a form of political and cultural control.
Roth v. United States is the landmark case where the Supreme Court rules that that 'obscene' material has no protection under the First Amendment. It thus created the one exception to free speech protections that doesn't involve demonstrable threats to the lives and property of others; instead, it's an exception involving nothing more than material which disturbs, offends, or upsets viewers. Insofar as "obscene" even has a definition, it's entirely a matter of the subjective standards of whomever is most offended — and when people are offended, it's frequently due to their religious beliefs about sex, women's social roles, bigotry, etc.
Because of the frequent efforts of religious conservatives to regulate anything involving sex, many cases dealing with such regulations end up in court. With this case, the Supreme Court laid down rules for cities' ability to create licensing, inspection, and zoning requirements for sexually oriented businesses. In the process, they ruled as unconstitutional the requirements created by the city of Dallas because they applied only to sexually oriented business and nothing else. This created a form of "prior restraint" based on the content of speech and depended solely upon traditional religious beliefs about the immorality of certain sexual activities.
Freedom of speech means little if it only applies to people which the majority likes. In Cuffley v. Mickes, the Supreme Court ruled that even Ku Klux Klan should be permitted to join an Adopt-A-Highway program, just like any other organization. The significance of this case is that it helped establish that once the government creates any sort of general benefit or program, it cannot excludes certain groups because their religious or political views are objectionable. This helps protect the ability of everyone to express their views freely and associate with others freely independent of government control, direction, or influence.
Both "pornography" and "protect the children" are frequently rallying cries of Christian Right censors. These two principles came together in the the Child Pornography Prevention Act (CPPA) which the Ninth Circuit Court of Appeals struck down. According to the Court, the CPPA violated the First Amendment, particularly with regards to the attempt to ban 'virtual child pornography'. The CPPA banned certain material not based on any actual harm it caused, but rather based on people's fears about what sort of harm it might cause, bypassing traditional standards of "clear and present danger." This ruling denied that the state could censor material based on religious or political fears.