"They … do not prosecute child pornography cases unless those children are being made into victims. They are being sexually assaulted," said Levenson, who was once a federal prosecutor.
Still, just the hint of controversy can have a chilling effect on museums and galleries.
A national debate was triggered in 1990, when the Contemporary Arts Center in Cincinnati exhibited the sexually provocative and explicit photos of Mapplethorpe.
The museum was prosecuted on obscenity charges. Though the center and its director were acquitted, the exhibition sparked a national debate about the public funding of what some considered offensive art.
That same year, authorities in San Francisco raided the studio of Jock Sturges, an accomplished photographer whose work has hung on the walls of the Museum of Modern Art and Metropolitan Museum of Art in New York.
Sturges' work includes studies of nude boys and girls. A grand jury declined to return an indictment, but in 1998, Sturges was again at the center of a censorship storm when attempts were made to prosecute bookstores in Tennessee and Alabama that were carrying his photography books. All charges were eventually dropped in both cases.
Celebrated photographer Sally Mann featured nudes of her children in a critically acclaimed collection of stills called "Immediate Family."
But in 1992, vocal critics condemned Mann's work as pornography during an exhibition at a New York gallery.
Museum and gallery directors admit these can be challenging times when deciding how to walk the line between being compelling and educational and offending community sensibilities.
"You have segments of society with very distinct and valid concerns about art and what's presented in art that might be tremendously offensive to other sections of society," said Elizabeth Merritt of the American Association of Museums.
"You have to be aware of both of those interests in deciding what you are going to present."
Merritt also says these debates are not always about sex or pornography. The "Body Worlds" exhibit drew criticism and created controversy because the riveting display of preserved real human bodies included fetuses.
"Museums are in the challenging position of being places that both raise interesting intellectual and aesthetic questions that try to make people think, but at the same time, safe places that people want to come to be involved in this ongoing discussion. It's a real balancing act," said Merritt.
Neither the gallery in Northern England nor the police are providing any further details of the investigation. It remains to be seen whether Goldin or John will face prosecution or penalty.
History may be on the artist's side. In 2001, police demanded a photograph of Goldin's on exhibit at Saatchi Gallery in London be taken down. The gallery refused and the authorities did not pursue the matter, reportedly because they felt a successful prosecution was not likely.
And there may be a silver lining for Goldin if the past controversies of other provocative still photographers are any indication.
The value of Sturges' nude photographs reportedly skyrocketed after his brushes with pornography law, making him millions.