Cock, Theatre Upstairs, Royal Court, London

A brilliant study in bisexuality

It's been a big week for the male member on the London stage. The penis is a wayward minx and, as Alan Bennett's Auden puts it in The Habit of Art, it's a shocking little shape-shifter, often shrivelling up so small as to make the distinction between circumcised and uncircumcised a matter of absolute speculation, while constituting a key instance of "the propensity of the flesh to creep".

Now, the young playwright Mike Bartlett gives us, on behalf of his generation, an equivalently brilliant and blackly hilarious feat of provocation (and one which also pivots on the penis) in Cock at the Theatre Upstairs. Trust me: you won't find such marvellous moment-by-moment acting (from the four-strong cast) or more punctiliously expressive direction (from James Macdonald) anywhere in the country at present. Admittedly, Cock is no prick-teaser as a title, but by the end of this intense, unbroken 80-minute encounter with Bisexuality and its Discontents, you may feel that word also connotes a load of overbearing, pernicious bollocks and richly describes the exclusively gay half of the conflicted young male, same-sex couple at the centre.

Lady Bracknell would have the right expression to encapsulate what John (the lovely, quiveringly subtle Ben Whishaw) is up to. She would say that he is "shilly-shallying". He's in a festering long-term relationship with M who is portrayed with acute comic understanding of the character's hang-ups by the ever-brilliant Andrew Scott. I'm afraid that I watched M with appalled fascination for the way he treats poor John is more or less how I treat my wife and children. His basic unit of communication is the kind of baroque aria of the bullying, queeny self-display that is self-dislike turned inside out. To admit anything to this man is to give a hostage to fortune; tell him that a woman is not your standard idea of feminine, and he'll turn it into a vicious running gag whereby the woman is incrementally transformed into Arnold Schwarzenegger with illegal levels of testosterone. Me all over, too, I fear.

John falls in love with a woman (a delicious portrait of an intuitive sounding board turning herself into a predatory guilt-trip from the divine Katherine Parkinson). John's affront to gay good taste really separates the men from the boys, so to speak. There's all hell to pay, and quite a lot of purgatory to be going on with. Up to now, though, this review hasn't been quite straight with you. You may be thinking that Cock is the bisexual classic of our era. But instead it eventually smashes through the notion of identity politics, arguing that they were a useful fiction on the march to liberation – a debate that flowers into dotty, divisive life when, bringing with him the prejudices of his own generation, M's father (all hilarious, low-key fixation in Paul Jesson's spot-on) arrives to back him up at a grisly dinner à quatre.

I may have given the impression, too, that the play is conventionally staged. In fact, it rinses the stage bracingly clean of all realistic redundancy, reducing – or rather aggrandising – the relationships to twitchy sport-like confrontations of mimed, fully clothed skin-flick nakedness in a wooden cockpit, with the characters talking in ways that often amount to nothing more than a running commentary on the emptiness of life in the hermeneutic rat-run of their self-making. And here is where the thematic and spiritual overlap with The Habit of Art is uncanny. Bennett argues that innocence is a myth and tolerance should come from a due recognition that in our hearts and in our imagination, we've committed most of the things that the world punishes. Bartlett contends that "thinking" in all the old customary categories is precisely what does our heads in. Both plays are seriously radical. Cock is also a heady example of the way that concentration on theatre's unique aesthetic capacities can help a dramatist to go the extra mile morally. I await Bartlett's next piece in a mock-sweat of unseemly anticipation.

To 19 December (020-7565 5000)