The casual decadence of this taboo-breaking

If the deity that was one of the biggest jokes came from any other faith, we might be rather upset

The Son of God has returned to the South Bank this week. A matter of days after the annual performance of Bach's St Matthew Passion at the Festival, He is making an appearance at the National Theatre. On this occasion, he descends from the gods at the Lyttleton, represented by a large black man in a loincloth.

But wait. On closer examination, it is not a loincloth that he is wearing at all but some kind of outsize nappy. Soon all becomes clear. Jesus has returned in the person and in the costume of a man who had appeared earlier in the evening – a diaper fetishist who performed a show-stopping musical number called "I Wanna Shit My Pants". For this is Jerry Springer: the Opera.

Jesus's role in the opera has a sort of mad logic to it. Jerry has descended to hell and has been ordered by Satan to oversee the greatest confession show ever – one in which Jesus, and the Virgin Mary, will be required to apologise publicly to the devil. Since the alternative is to be sodomised with barbed wire, Jerry agrees. In the scene that follows, Jesus the nappy-man turns out to be pretty much as foul-mouthed and sexually confused as any other Jerry Springer guest.

At this point, something rather strange happened to me. I had been swinging along, enjoying this gloriously rude show with the rest of the enthusiastic first-night audience. Now I became aware of a twinge behind the amusement. It was not outrage, nor even active disapproval. But, perhaps because this also was also a religious moment portrayed in musical form, I thought of the Matthew Passion and, for a fleeting moment, I felt uneasy.

Not that a few seconds of queasiness spoilt the evening. A sense of liberation from tired old hang-ups of sex, class and religion attends performances of Jerry Springer: the Opera. All those grotesqueries, humiliations, outbursts of domestic violence and acts of audience bullying which so many of us have secretly enjoyed while watching TV confession shows from America have been transformed into an evening of hilariously foul-mouthed, camply outrageous mock opera.

The seedy is rendered grandiose, the sexually perverse acquires romantic pathos and a freak show revealing human misery and frailty under the glare of studio lights becomes a theatrical evening of wit and sophistication. Every turn and number is a deft celebration of sexual dysfunction, scatological obsession, stupidity and swearing – a bravura turn from a transvestite "chick with a dick", an aria from a lesbian dwarf diaper fetishist, a song-and-dance routine from members of the Ku Klux Klan.

Taboo-breaking is something of a hot ticket right now. Hundreds of people have queued from early in the morning to be given the chance to be photographed in a naked crowd scene at London department store. A Catalan bonk-fest called XXX is a sell-out at the Riverside Studio. It seems to have become accepted that at last we are grown-up enough to accept the glorious sexual variety of 21st-century man and woman, to recognise that virtually any expression of desire has its own validity, that we have all bounded free from the shackles of bourgeois morality.

On the other hand, it could mean that we are becoming morally slack, casually decadent. The laughs in Jerry Springer: the Opera (and they are very good laughs) are not at our own expense but derive from an easy sense of superiority. We revel in the humiliations to which the impoverished, badly-dressed, over-sexed, stupid members of the American underclass are prepared to submit in order to have what the show calls their "Jerry Springer moment".

If the theme of the opera were sexual perversity in Hampstead, or it was the British National Party that was tap-dancing on stage, or even if it starred our own home-grown underclass – a junkie from a council estate in Bristol, say, and a teenage hooker from King's Cross – the laughs would come rather less easily. If the deity that was one of the biggest jokes of the evening came from any other faith apart from poor old Christianity, we might be rather upset.

So the liberation of shows like this may give us a great night out but it is conditional and essentially heartless. Those whose ghastly, messy lives provide the raw material for the drama may be floundering about in a moral vacuum, but then so are we, the audience who laughs at them.

As a guest on Radio Four's Saturday Review, I have been lucky enough to attend two first nights. By coincidence, the second also touched on decadence. In the impressive Donmar Warehouse production of Camus' Caligula, there is more blood and fewer jokes than at the National, and the religious element is provided by a scene in which Caligula dresses up as the goddess Venus and forces his senators to worship him. "It was blasphemous," Scipio protests. "No," says Caligula, smiling. "It was theatre."

terblacker@aol.com