The Museum of Sex has held my curiosity ever since it first opened in 2002, though I had never been there before this week. Its name suggested to me a place made up of equal parts Museum of Natural History (fossilized artifacts), Kinsey Institute (the scientific study of human sexuality), and “cultural institution” (it has both a mission statement and a gift store); in other words, a combination peep show, PBS documentary, and porn shop in which, civilized and guilt-free, you can expand your mind and other parts of your anatomy.

To varying degrees, I was right on all counts. “Peeping, Probing & Porn: Four Centuries of Graphic Sex in Japan,” currently on view at the Museum of Sex, includes more than 100 works, some contemporary hentai (“perverted”) manga and anime, but mostly traditional shunga (“pictures of spring”). Shunga were graphically erotic woodblock prints produced by artists from the so-called “Floating World” that flourished during the Edo period (1603-1868).

This is a rare, no-holds-barred show inappropriate for anyone under 18. The works are very explicit: We see plenty of voyeurs, exhibitionists, prostitutes, rapes, masturbation, copulation, penetration, homosexuality, and bestiality. Fiery, enlarged, distorted, almost monstrous genitals are the central focus of many prints, seeming as if they were held for our scrupulous gaze under a magnifying glass.

The show, curated by Elizabeth Semmelhack, is long overdue and in many ways startlingly strong. Focusing on Japanese erotica during the period of isolation and exclusion, before 1853 – when Commodore Matthew C. Perry opened up Japan to the West – “Peeping, Probing & Porn” is weak on Meiji-period and early to mid 20th-century works. But it is probably a stronger show for it. There is a mediocre Western-inspired World War II image of an army nurse masturbating; Tezuka Osamu’s famous and influential book “Ribon no kishi” (or “Princess Knight,” 1954); and a substantial collection of cutting-edge late-20th-century and contemporary manga and anime. But the bulk of the show, both artistically and in number, belongs to the art of shunga.

Sex was one of the most popular themes among the ukiyo-e (“pictures from the Floating World”) artists. Most of the show’s images depict the nightly goings-on in Yoshiwara, the pleasure quarter of Edo (modern-day Tokyo), a gated, very expensive district where thousands of prostitutes and geisha (called “night cherries”) were available to bachelors in teahouses and licensed brothels. Every major artist illustrated shunga,alucrative business, but because laws prohibited their making, most prints were left unsigned to avoid prosecution, though their authorship was well-known. Here there are spectacular prints, a few of which are reproductions, attributed to Kunisada, Kuniyoshi, Kuniyasu, Utamaro, Utagawa, Eisen, and Hokusai.

Most works of shunga, which each cost roughly the price of a bowl of noodles, were cheaply made woodblock prints purchased as masturbation aids and thrown away after their one-time use. (You can imagine that many sheets of shunga – like the pieces of paper used for cleanup after sex and often depicted, strewn about the brothels, in the shunga prints – resembled today’s bedside boxes of Kleenex.) But every upperclass bride was expected to bring shunga as part of her dowry, so some of the prints are of very high quality, and most, because of their racy subjects, are rarely if ever exhibited in the United States.

The Japanese, before the influence of the West, always maintained a direct connection to nature in their art – not to the look of nature but to its actions. And sexuality is a force of nature if ever there was one. In the prints, we see tangled bodies, their limbs and genitals all akimbo, as they roll and unroll, fold and unfold, in brilliantly patterned, forward-pressing masses that suggest the violent movements of wind, water, and beast.

Sex is treated in the prints not as a chaste encounter, an impure act, or a fall from grace, but as a staged, animalistic performance.The prints suggest a cataclysmic collision of forces that resembles wrestling lions and snakes, spreading root systems, dams breaking, and volcanic eruptions. Grace, chaos, and lust – bound in beautiful, bright-colored wrappings – are held in absolute erotic tension.

When East met West, the ukiyo-e prints, with their almost abstract flat patterns, overactive surfaces, close cropping, and everyday subject matter, liberated such artists as Degas, Toulouse-Lautrec, and van Gogh. But the golden age of shunga died with that of the brothel and the Floating World, which officially ended with the passing of the Prostitution Emancipation Act of 1872.

The Museum of Sex is doing some welcome and necessary work, and “Peeping, Probing & Porn” brings some of that old Japan back to life. But the exhibition’s sideshow installation, which emphasizes the artworks’ “porn” aspects over those of erotica, ultimately robs the images of some of their poetic richness.

Entering the lobby of the museum is like walking into a cheap movie theater. As soon as I handed my ticket to the guard, passed through the metal turnstile, and walked into a pitch-black tunnel, my expectations dwindled along with my eyesight. I wondered: Was I in a haunted house or a museum?

As my eyes adjusted to the darkness, I was drawn to black walls punctuated, salon-style, with glowing rectangular openings from hip- to headheight. The holes were roughly the size of each print, and viewing required that I stand very close (as if “peeping” at the works) and that I stand on tiptoe or bend over completely – a position that, at a show of sexually explicit material, felt somewhat compromising.

Granted, works on paper are delicate and require small amounts of light, but the gallery is too dark by any museum’s standards. The installation is unfriendly to anyone of less than average height, and it does not allow for comparisons or distant readings of the works. The show, which puts art at the mercy of its thematic installation, beats the dead horse of its “peeping” and “probing” metaphors and makes the museum antithetical to the show’s wonderful images.

At the Museum of Sex (233 Fifth Avenue at 27th Street, 212-689-6337).