the courts have consistently held that the right to free expression does not extend to material which is obscene.

Freedom of expression has contributed much to the development and wellbeing of our free society. In the exercise of the First Amendment right to free expression which everyone enjoys, sex may be portrayed, and the subject of sex may be discussed, freely and publicly. Although material is not to be condemned merely because it contains passages or sequences that describe or depict sexual activity, the courts have consistently held that the right to free expression does not extend to material which is obscene.

http://www.usdoj.gov/criminal/ceos/obscenity.html