Miller v. California
Supreme Court of the United States
413 US 15
June 21, 1973 Decided
http://www.law.umkc.edu/faculty/projects/ftrials/conlaw/miller.html

Let us quote the US Supreme Court obscenity standard in the above case that changed the way we view US porn:

The basic guidelines for the trier of fact must be: (a) whether "the average person, applying contemporary community standards" would find that the work, taken as a whole, appeals to the prurient interest, (b) whether the work depicts or describes, in a patently offensive way, sexual conduct specifically defined by the applicable state law; and (c) whether the work, taken as a whole, lacks serious literary, artistic, political, or scientific value.

The above standards is the same reason why old US porn have screenplays that show porn actors and actresses reciting passages from the works of William Shakespeare. It is also the same reason why American dicks in old porn are limp compared to Pinoy dicks that seemed to be rock hard.

If you read the decision, you will be surprised that the Supreme Court has humbly accepted that it is difficult for them to define what is obscene. Hence, let us quote that paragraph here.

We deal with highly emotional, not rational, questions. To many the Song of Solomon is obscene. I do not think we, the judges, were ever given the constitutional power to make definitions of obscenity. If it is to be defined, let the people debate and decide by a constitutional amendment what they want to ban as obscene and what standards they want the legislatures and the courts to apply. Perhaps the people will decide that the path towards a mature, integrated society requires that all ideas competing for acceptance must have no censor. Perhaps they will decide otherwise. Whatever the choice, the courts will have some guidelines. Now we have none except our own predilections.

Please note the reference to the Bible's Song of Solomon.