Opposites attract in Lucy Foster's lightweight love story – until, that is, they don't. J and K might be a same-sex couple, but in every other regard, they're almost the exact inverse of one another. J's sunny, K's stormy; J's wet, K's dry as hell. One looks to the future, the other lives for the present. They're light and dark, chalk and cheese, you say 'potato', I say ‘otatop.' It was never going to work. Was it?

Narrated in hindsight in front of big gold balloons that spell 'HAPPY F**KING WHATEVER', Lobster lets us know the answer right from the off. Bitter notes creep into even its tenderest moments and, from the moment they meet in the middle of a house party, struck dumb by mutual attraction, we know that J and K are doomed to split up. The Monkees' chipper lyrics – "I thought love was only true in fairytales" – swim round the room, and for 90 minutes, Foster dares us to believe despite their inevitable incompatibility.

Because for all their differences, J and K somehow seem to click. At their best, J softens K's icy cynicism, while K proves an outlet for J's over-enthusiasms. They see the best in each other (against the worst in themselves) and maybe, just maybe, one might rub off on the other. As J confidently mis-speaks: "The world is our lobster."

Foster's point, perhaps, is that all things aren't possible. As the relationship continues, cracks creak in and the question is whether the big fundamentals will break it before the minor irritations do. As J pushes for marriage, mortgages and kids, K bristles at commitment. Would they be settling down or just settling for?

However, Lobster doesn't tell us too much about the way modern relationships work. It's so strenuous in setting up two polar opposites that Foster's characters could only ever be fiction. For all she sews them into the world with banal, everyday details – Tesco Express trips and Buzzfeed listicles – both women are little more than character traits made flesh. Foster gives up on reality for the sake of her rom com and, in the process, fudges both rom and com by aiming at each too directly.

Love, in drama, is a delicate thing. You have to sneak up on it – and it has to sneak up on you. Plus we have to believe in people to believe they should be together. Look at Beginning, David Eldridge's brilliant real-time romance headed for the West End next week. It coaxes love into life like kindling, precisely because it's not the be-all-and-end-all. Eldridge's hesitant, fumbling lovers have lives of their own. They talk Scotch Eggs and Strictly, families and pasts. Foster's, by contrast, only make sense as each other's opposite number. They talk, mostly, about their relationship.

Kayla Feldman doubles down on the writing's tiggerish energy, and there are characterful, albeit over-zealous, performances from a sure-footed Louise Beresford and a goofy Alexandra Reynolds, but none of that disguises an overcooked love story. Lobster's no delicacy.

Lobster at Theatre503 until 20 January.