The Girlfriend Experience, Young Vic, London

The "girlfriend experience" is what you get in a Bournemouth brothel in exchange for upwards of £40. If there's a gnome on the front doorstep, the ladies inside are well busy. If not, step right up for massage and hand relief, oral or full, dressing up, spanking and water sports.

Alecky Blythe's play, first seen last year in the Royal Court's studio, is a 90-minute piece of "verbatim" theatre assembled from more than 100 hours of taped conversations between Blythe and the prostitutes she visited. One of them, the elephantine Tessa (Debbie Chazen) giggles nervously about going to see a play that she's in.

There's no point in not saying that these huge and unappetising women come across as being incredibly sad and curiously stupid, with their whale-like lolloping around, matter-of-fact disposal of satiated condoms in the sitting room wastebasket and quest for affection despite affirming that they would never date a client (two of them do, finally).

Blythe's technique of recorded delivery – slavishly followed by her director, Joe Hill-Gibbins – is to play the script through the sound desk into the permanently in-place earphones of the actors. They never remove them. It is said they never learn the lines but regurgitate the play as it comes to them down the wire. The sound engineer sits at the side of the stage.

I just don't get it. The fresh and spontaneous-seeming performances – and they are certainly that – strike me as coming from the stage despite this rather ludicrous handicap. There's no attempt to suggest some sort of interesting dislocation between the actor and the material; all four girls are well defined characters battling against the depersonalizing process.

As well as Tessa, who has a 16-year-old daughter, there's Lu Corfield's mammoth Poppy who seems permanently zonked out in red knickers and skewwhiff purple wig, declaring she's got a couple coming on Friday ("all our girls are genuinely bisexual") as if they were a garden manure delivery, and imbibing pints of fluid to facilitate a urinary service; Esther Coles's bare-bottomed Amber staving off her husband on the phone and obliging all-comers (sic); and Beatie Edney's gurgling, optimistic Suzie cramming her face with sandwiches and her fuller figure into a nurse's uniform.

The punters come and go, literally, in the single shape of Alex Lowe, who is known to the girls by his various names of Viagra Man, God's Gift, Groper or just plain Mike. At one point, Lowe plays a first-time client who tries to begin his appointment in the hallway with Poppy while Mike the toff is inside with Tessa letting off all sorts of hyena-like sounds of orgasm.

Is this funny, or is it just mucky? Both, probably, but I don't think an argument against this being degradation and exploitation is easily mounted (oh dear, sic again) by saying that a) the girls are happy in their work or b) their choice of it is a sign of liberation, not repression. Blythe's play is nonjudgmental but I was still left feeling queasy and uncomfortable by its moral neutrality and sexual explicitness.

Lizzie Clachan's design is a free-standing arrangement of balsa-wood flats, as if to suggest the improvised nature of the performance (the biggest deception of all) and the impermanence of the "sea-front drop-in service." Clutching teddy bears with their dildoes, these big girls in big underwear are really just social flotsam, kidding themselves into dreams of belonging.

The prostitutes talk to us in the interview-speak of Blythe's transcript but it's not clear if the exchanges between them – as to who, for instance, is going to pull on the leather gloves to go and stimulate the OAP who's just hobbled, bent double, into the parlour – are similarly authentic.

To 15 August (020-7922 2922; www.youngvic.org)