The silver drinking cup dating from around AD50 features a bearded man, a youth of around 12 and a younger boy.
The museum will be putting the cup on display from today. Visitors who might fear accusations of voyeurism can be assured that they will be brushing up on classical knowledge. Dr Dyfri Williams, keeper of Greek and Roman antiquities at the museum, said the cup depicted "two scenes, each set indoors with two males sharing a mattress". But he was quick to add for scholars that "the wreaths worn by the youths suggest that they had received an education derived from classical Greek principles".
The Heritage Lottery Fundpreviously turned down the museum's request for pounds 1m for the goblet. Its press release said only that the cup "depicts two erotic scenes worked in relief". The National Art Collections Fund, which gave pounds 100,000, was less coy. It said: "Now that a more open- minded attitude towards the culture of the past prevails, the piece can at last take its proper place in the study and appreciation of early Roman silver."
If that all seems a long way from Latin lessons, it turns out that the goblet has a racy recent history as well. Until 1928, it was in the collection of American Edward Percy Warren, who also commissioned Rodin's sculpture The Kiss. Warren lived at Lewes House in Sussex. Dr Williams said: "Warren's household of mostly men saw Plato's Symposium [about love and friendship] as their charter. They referred to this cup as The Holy Grail."
The Roman drinking cup goes on display today