David Atfield’s Exclusion is an artful piece involving a messy political and personal "what if?".

What if a prospective prime minister was married but also having it away with a gay staffer? What if the same staffer could be let loose to try to seduce the prospective PM’s rival?

It’s a comedy set in the "now" of legal same sex marriage, with much of the humour coming from the need to navigate new and shifting sexual seas. But issues like love and loyalty might be stumbling blocks. And marriage is one thing – but what if the other woman is another man?

Jasper Ferrier (Craig Alexander) takes the view that he is married and so therefore can’t be gay. His relationship with Craig (Ethan Gibson) is simply some kind of fling. Craig might not see it as so lightweight. Neither might Jasper’s wife Jacinta (Fiona Victoria Hopkins), if compelled to confront matters directly.

Rival Michael Connor (Michael Sparks) is in a longstanding and somewhat subdued marriage with Caroline (Tracy Bourne). She’s religious. Both seem sexually reluctant but happy enough together.

The result is a somewhat melancholy comedy, with Jasper’s blindly entitled attitudes and Craig’s unacknowledged relationship with him at the centre.

A fine cast tackles all of this with sensitivity and humour. Alexander nails Jasper’s selfish failure to recognise his own nature. Hopkins is deeply believable as the pragmatic Jacinta. Bourne makes of Caroline a credible and sensible character, blinded to the truths about her marriage by sincerely held religious beliefs. And as the young man at the centre of the story, Gibson has presence and perception.

It’s the development of Spark’s slightly wistful Michael, however, that becomes a moving focus as the play progresses.

The frequent nudity is relaxed and exuberant. Imogen Keen’s straightforward set that combines images of beds and desks is a good metaphor for the goings on. A half seen TV set behind a gauze curtain seems to be continually on a half heard, half seen newsfeed and occasionally characters pause there as if observing the main stage in a way that constitutes an unstated but occasionally powerful commentary.

Atfield directs his own play with geniality and perception but the piece sets up a pattern of short filmic scenes, which sometimes feel like they could use extension. A sitcom feeling threatens but depths do arise and there’s a truthful questioning of the characters’ dilemmas.

Ironies abound. It would be a shame to reveal more.