Nothing is more likely to inspire us to see for ourselves than a warning about the effects of looking. Take the media interest this month when it was revealed that the British Museum's exhibition, Life and Death in Pompeii and Herculaneum, is to include a "parental guidance" notice. The reason? An ancient marble sculpture of the god Pan (a part-human, part-goat figure) having sex with a she-goat is not to be segregated, as it has been since its discovery in 1752, but displayed openly with the other exhibits – a liberal move by London, if also one which dulls the object's impact. Getting this story into the news ensures that centuries of censorship are not swept under the carpet, and that Pan, and the show he speaks for, remain "hot property".

But the news story also exaggerates this censorship. Far from being forgotten in its first modern home in the royal palace at Portici on the Bay of Naples, the sculpture, which was part of a restricted collection in the cellars, was quickly a celebrity.

The German art historian Johann Joachim Winckelmann, who visited the palace four times between 1758 and 1767, thought better of being "the first to apply" for a viewing licence, and called the Englishmen who claimed to have seen the piece liars. But by 1794 another Englishman is insistent: he writes that he has seen the sculpture, but that it is too indecent to describe, deserving to be thrown into the volcano. Luckily no one listened, and in the 19th century the sculpture was transferred to its new home, now the National Archaeological Museum of Naples, where it again became part of a reserved collection.

For anyone still too shy to seek permission or bribe the guard, there were published engravings and photographs. In 2000 the museum opened the "Secret Cabinet" to a fanfare of press (and to women as well as men) but, by keeping the cabinet as an entity apart, still branded Pan and the goat "pornographic".

What would the inhabitants of ancient Herculaneum and Pompeii have thought about the fact that, of some 250 objects representing them in the exhibition, which opens on Thursday and runs until 29 September, Pan and the goat should be the trailblazer? How typical is it of the sculptures that surrounded them on an everyday basis?

More typical, certainly, than it is for us today. Turner prize winner Mark Wallinger's plan to erect a colossal sculpted horse in Ebbsfleet, Kent, has proved controversial enough – imagine the furore if he had wanted to have this horse mounted by a second steel-and-concrete stallion or a fibreglass humanoid! Sculpture, and sexually explicit sculpture at that, some of it violent, was common currency in Roman culture across all sectors of society –and crucially was on open access.

Beyond sculpture, sex scenes graced drinking cups, oil lamps and dining room walls, and tintinnabula, or phallic wind-chimes, were suspended from doorways.

A public bathhouse just beyond the walls of Pompeii encapsulates the difference between ancient and modern sensibilities nicely: above each locker in the building's only changing room was what we might call a top-shelf picture. One scene shows a threesome of two males and a female, another a man performing cunnilingus on a woman, and so on.

Normative these are not: whatever the Romans were up to in the bedroom or the brothel, their moral discourse – as ancient graffiti, satirical poetry and prosecution speeches make clear for us – condemned oral sex as filthy, particularly when it was the man who was giving the woman pleasure, and especially if, as in the image, it demanded that he squat on the floor in front of her. But this was the bathhouse's only changing room, used by women as well as men – these pictures were acceptable even if they were risqué, sideways glances at sex and gender in a space which was all about the body, and funny reminders of where one had left one's tunic (imagine a wealthy woman explaining to a slave where she had left her clothes.

But locker-room humour cannot explain all Roman erotica – if indeed "Roman erotica" is a tenable category. Some of the sex scenes in wealthy houses at Pompeii are just too pretty to be funny, their exquisite painting and expensive pigments signalling seduction rather than obscenity.

Tintinnabula, meanwhile, were designed not to attract the viewer but to ward off evil, sometimes by arousing raucous laughter but often something more sinister. Whether an erect penis was seen as a sex organ, marker of masculinity, fertility symbol, or weapon depended on the context.

If we are to understand Pan and the goat, we have to understand that Pan is a god. The boundary between the sacred and the secular was rather more porous in classical antiquity than it is today.

So in an intimate room in one Pompeian house two panels, each showing a man and woman enjoying sexual intercourse, share the space with a painting of the demigod Hercules. The gods were notoriously incontinent when it came to their sex lives – not simply full of the emotions and urges that their anthropomorphism demanded, but excessive in these emotions and urges, as befitted beings who looked like mortals but were superhuman.

A favourite scene, both in frescoes and on marble coffins or sarcophagi, shows the sleeping shepherd boy Endymion being visited by the Moon goddess Selene, who bore him 50 daughters. Communion with the divine is here configured as sexual intercourse.

But assuming human shape was just one option for a god. All of them could shape-shift – the king of the gods, Jupiter, preferring to appear to his mortal consorts as an animal. The reality of his divine form was, as Bacchus's mortal mother, Semele, found to her cost, impossible to countenance: encouraged by Jupiter's jealous wife, she asks him to reveal his true self, only to be burned to a cinder. Jupiter manages to maintain a swan-like disguise in his sex with a second mortal woman, Leda, causing her to give birth to her children in eggs.

But images of their lovemaking, popular across the ancient Mediterranean, are still difficult to look at. And so they should be: for it is epiphany that we are witnessing here, the moment that divine power, in all of its unsettling strangeness, makes itself manifest. What would it mean for us not to be taken beyond our comfort zone in seeing and feeling the gods?

Myth held that the Theban seer Teiresias was blinded, and the hunter Actaeon turned into a stag and ripped apart by his dogs, as punishment for stumbling upon a naked goddess.

Where does this web of human and divine experience leave Pan and the goat?

Made from a single piece of Italian marble and highly polished as befits a work of quality, the small sculptural group was originally displayed in a large waterfront villa now known as the Villa of the Papyri, just outside Herculaneum. This first-century BC villa is as renowned for its extraordinary library (the charred remains of which have given the site its name and scholars endless work in piecing together lost examples of ancient learning) as for its sculpture collection.

Pan was found in the garden, south of the villa's pool, together with three marble herms, one of them a portrait, often identified as a successor of Alexander the Great, Demetrios Poliorketes, who ruled Macedon in the first quarter of the third century BC; the others were philosopher types with frowning brows and beards. Throughout the garden, images of athletes, thinkers and dynasts vied for attention with female figures, gods, animals and satyrs.

Pan and the goat lie at the very limits of this range: if this garden is designed to stimulate visitors to talk philosophy with the owner and to leave the banalities of everyday life behind, Pan and the goat threaten to puncture the Arcadian idyll.

Except that Pan, the son of Hermes, whose statue is also in the garden, is the veritable god of Arcadia. This is his epiphanic moment and one which puts the viewer's mortality under the spotlight: is Pan more like him or like the goat? Is the viewer closer to the gods or to an animal?

Funny this sculpture may be, but it also hits hard, urging Romans to think harder about what it means to be male and female. It enables us today to access ancient life, not just ancient sex lives.

Not that these lives were any more licentious or uniform than ours (shortly before the eruption, the scenes from the baths were painted over). But ancient lives were certainly different. We will never see what the Romans saw. But, with parental guidance, in the act of looking we can learn more about what it means to be human.

Carrie Vout is a senior lecturer in classics at the University of Cambridge. Her next book, Sex on Show: Seeing the Erotic in Greece and Rome (British Museum Press), is out in August.