This article is from the Sex Movies FAQ, by director@gti.net (The Director) with numerous contributions by others.
Well, it's not really illegal... but it is one of those things that
can be easily "nuisance prosecuted."
The American laws do not restrict any specific type of sexual
activity other then pedophilia a/k/a "kiddie porn." HOWEVER, the
Supreme Court did have a say in what was considered obscene. Bill
Majors says:
Just to give you some idea of what the Court thinks is
obscene, masturbation is listed as an "ultimate sex act." it
is that definition that leads us towards what may and may not
be prosecuted.
However, the fact is that when the government decides to
prosecute the filmmaker or manufacturer it is always on the
losing side. A federal trial (and that is what most of them
are) will cost from 250,000 to over 1 million bucks with no
guarantee of the outcome. This is why so many folks make plea
bargains, spend six months or so as a government guest and
then we have the funny side of everything. Usually a large
fine is levied against the corporation, but how do the figure
the corporation will make the money to pay the fine? Not by
making kids shows, but by making more of what they were just
convicted of. So in effect the government becomes a partner in
the porn business.
The very first adult films were documentaries and quasi-
documentaries packed with 'redeeming social value' so as to prevent
the possibility of convictions for obscenity (under the three
pronged test for obscenity set by the US Supreme Court (Miller v.
California):
1) appeals to the prurient interest,
2) patently offensive because it affronts contemporary community
standards,
3) utterly without redeeming social value).
The words "community standards" have become two of the most
ambiguous words in American case law.
As you would imagine, a town in the Bible Belt has very different
feelings about porn than a major metropolis like New York City or
San Francisco. So, while fisting may be valid by New York's
community standards, it probably wouldn't fly in Topeka.
So now you have different parts of the country tolerant of
different things. Basically, it is a hassle to keep track of all
the places that certain acts are "good" versus "bad." So, the
industry self-censors itself, and leaves out the really raunchy
stuff in their domestic releases. (Europe where they have their
heads on straight about sex, is a free for all sexually, except for
kiddie porn (though their age of consent is lower than ours)).
Nick Long says this about the fisting guidelines:
For clarification purposes and the information of the curious,
during filming of any insertion scene coming close to fisting, the
general rule to avoid legal complications is that
'the thumb must be visible'
Number of fingers inserted is immaterial, provided all 5
digits do not disappear inside the plane of the lips or
sphincter ring.
That appears to be a production stage rule, not necessarily what
any one state or local jurisdiction might rule in court.
An interesting tidbit from Peter van Aarle: On some sets (usually
when Bionca and Debi Diamond play together ;-) fisting still
occurs, and IS still filmed, but the footage is never released for
US versions anymore (but often still IS released in Euro versions).
Tim Evanson says that (surprise!) things are different on the gay
side:
Fisting is increasingly common in gay videos, and in a few
videos items such as shampoo bottles, beer bottles, and even
traffic cones may be inserted into a man's anus. Most gay
porn companies deal with the subsequent legal problems by
editing the offensive scene out of the movie entirely. Many
mail order companies offer a "mail order only" version which
includes the fisting scene, while rental and local-sales
copies of the movie do not have it included.
 
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