@ Traverse Theatre, Edinburgh, until Sat 27 Feb 2016

That impenetrable equation LGBTQIA where B = bisexual seems like a periodic table all of its own. It’s refreshing to find, in gay History Month, a play that is not about who does what to whom but how they feel inside.

John (James Anthony Pearson) is splitting up from his partner (Johnny McKnight). Is it just a seven-year itch or something more existential? “We are fundamentally different people,” says John then discloses that he has slept with a woman (Isobel McArthur) he sees regularly on the way to work. The two men circle each other on a bare stage lit like a boxing ring. The immature John brings a toy teddy as a ham-fisted peace offering. This does not go down well. His ideal husband thought they’d stick together no matter what.

It’s fraught. It’s real. It’s often uncomfortable and often very funny. “You’re lucky to have me,” yells John with an arrogant flourish drawing a gasp from the audience. It’s one of the things you don’t say when a relationship goes bad. Mr Boyfriend admits that it’s okay to like both men and women “but not at the same time!” This dance of relationship death is sometimes made literal with uneasy physical movements from the cast used as punctuation marks.

What’s really under the microscope is petulance and vacillation and inability to articulate complex feelings. John loves two people but – through weakness or shallowness – can’t decide which to share his life with. But whoever said you can’t love more than one person at a time? It seems as if he’s prepared to throw off a long-term relationship like last year’s iPhone. Amid the anger, hurt and misunderstanding the (unnamed) woman says of John “there’s a lot of emotional crap floating about you like space junk.” As expected from Mike Bartlett, the writer of the hit King Charles III, the intelligent script positively sizzles. The fast-talking Johnny McKnight has the best lines saying all the things anyone who has been cheated on would love to have said.

To resolve matters the party of three get together for a damage-limitation dinner. Only there’s a fourth (Vincent Friell). John has to face his fears and decide who he’s going to stay with. The play – a Glasgow Tron production which was originally seen at London’s Royal Court in 2009 with Ben Whishaw playing John – sidesteps handwringing gestalt. It’s a thought-provoking and often hilarious rumination on how we live now. This is not a “gay play”, is not even about bisexuality per se, but about the ache of longing of facing up to consequences, about how people treat each other and the true meaning of trust and its limits.

“Gay and straight are words our parents made up,” says the woman. The old rules no longer apply. The ancient Romans made no such distinctions. It’s something the late Gore Vidal would have sympathised with. He believed in the fluidity of sexuality – yet all his life he claimed to be bisexual when everyone knew he was gay.