Alana's often sorry after sex. She apologises over and over. Her body won't let her. It screams out in pain whenever she attempts penetration. She can't even do it herself. Her vagina keeps its lips shut and so, out of shame, does she.

Opening the new Bunker theatre, a converted car park beneath the Menier Chocolate Factory in Southwark, Isley Lynn's play is a tender, intimate and frank account of a young woman's sexual history. Alana (Lydia Larson) talks us through its start: a decade of awkward fumbles and agonising aborted fucks. It's not unlike any other account of teenage sex: a string of firsts – periods, snogs, dates, boyfriends, and the painful first time; all thrilling and horrifying at the same time.

For Alana, however, that first pain never eases up. It screeches up on her again and again, so intense that she sometimes faints and fits in bed. She can't use tampons or masturbate internally. Vaginal penetration seems impossible. It becomes her Everest.

Lynn's writing lets cringe comedy pull against Alana's emotional distress, and Larson gives a performance of real suppleness. She conceals her shame beneath a smile, a conveys a real depth of isolation in reasoning herself either incompetent or abnormal. On an increasingly disheveled double bed, she sits in flesh-toned underwear, emotionally naked, while her two castmates get the dignity of dungarees. Jessica Clark, a fine caricaturist, is very funny as Alana's eavesdropping mother, too mortified to talk birds and bees, and her box-ticking best friend – a self-defined sexual conquistador, while Jassa Ahluwalia plays her various sexual partners with comic sensitivity, never heaping blame onto their individual shoulders.

Instead, Lynn's target is wider: a whole social attitude. Alana has vaginismus, a psychosomatic condition that causes vaginal muscles to seize up on contact. It takes her until 20 to consult a doctor for a straightforward explanation and, after more shame and celibacy, another five years for anyone to tell her that it's fine, that vaginal intercourse isn't the pinnacle of sex, and that her sex life need only satisfy her. There are, as they say, many ways to skin a cat.

Lynn's is a campaigning play: one that pushes back against a euphemistic culture and prescriptive sex education. Its kickback against normative notions of sex is a liberating thing. However, fist-bumps aside, it's a theatrically naïve: too long and repetitive, too little variety or technique. All lot of its titillation - Adrian Mole meets Embarrassing Bodies - and Lynn's warts-and-all frankness leads to exhaustiveness, and the play's point about the need for up-front discussion lands long before Blythe Stewart's staging gets its end away.

Skin a Cat runs at the Bunker Theatre until 5 November