Japan's first major shunga (literally, "spring pictures") exhibition of erotic paintings and woodblock prints, is surprisingly hard work. As a venue for a ground-breaking assembly of images, which probably would not have been shown publicly if it were not for a highly successful shunga exhibition at the British Museum that opened in 2013, the relatively small space of the Eisei Bunko Museum has been seriously packed with visitors, making it tough to get a good look at the exhibits.

By "good look," I don't necessarily mean a sexually satisfying, lascivious, voyeuristic gawp, but a reasonable length of time in which to consider the complexity of many of the images. Japanese erotic prints have also been called makura (pillow), nure (wet) and warai-e (laughter pictures) and, depending on the artist, era and audience, they can be intentionally artistic, instructional and/or comic. To call them pornographic, considering that, by definition, they revolve around sex, has not been as easy as one might suppose.

The British scholar Timon Screech caused a splash in the late 1990s by suggesting that they could be called "pictures to be held in one hand," cutting through a history of elliptical discussion, by older heterosexual male academics that focused on the aesthetics and intertextuality of images in which just about every conceivable variation of bumping uglies can be seen.

There's male and female homosexuality, bondage, pederasty, rape fantasy, sex with ghosts, bestiality, threesomes, you name it. The sex part is astonishing, but it is also surprising how much interpretation is needed to unpack the images. Despite genitalia traditionally being exaggerated in size and depicted the same size as heads, a stylistic feature that has been described as a deliberate contrast of private and public "faces," the elaborate and semi-abstract tangle of limbs and kimono folds that characterize shunga are a visual puzzle that hide audacity in plain sight.

There are also puns, parodies and allusions to decipher. The title of Utagawa Kuniyoshi's "Omi Hakkei" ("Eight Views of Lovers' Meetings"), for example, is a homophonic and visual play on a series of ukiyo-e from the Omi region, which reappear on fans in each corresponding image of Utagawa's shunga prints, and which themselves were based on a series of classical Chinese landscape paintings.

Not all shunga, however, are so sophisticated. The Eisei Bunko exhibition features later mass-market mame-ban, ("bean-sized" prints) destined for Japanese soldiers of the Meiji Era (1868-1912) despite increasing censorship at the time. These later images are rougher in conception and execution, with the woman more often shown as the passive recipient of aggressive male lust. This change in temper adds yet another layer of reading to these fantastical images, as it illustrates that sex is not just what comes naturally, but subject to social and cultural change.

"Shunga Exhibition" at Eisei Bunko Museum, Tokyo, runs until Dec. 23; 9:30 a.m.-8 p.m. (Sun. until 6 p.m.). ¥1,500 (over 18s only). Closed Mon. www.eiseibunko.com