Last week, a very different period drama hit cinemas. Professor Marston and the Wonder Women concerns a real-life love story between a professor and his academic wife – and their teaching student, Olive. From the late 1920s onwards, they begin sharing a workplace, a bed, a home and eventually a family.

Angela Robinson’s biopic of the creator of Wonder Woman, American psychologist William Moulton Marston (Luke Evans), may be the most positive depiction of polyamory – the state of being in love with more than one person – in mainstream film to date. It posits that the comic-book superheroine was inspired by a happy, long-term union between the feminist Marston, his brilliant, acerbic wife Elizabeth (Rebecca Hall) and bright young student Olive Byrne (Bella Heathcote), as well as their dalliances with S&M, a theme that worked its way into the comics. Despite the controversy the latter caused at the time, it is an accessible, occasionally moving film that treats the three-way relationship much like a typical movie coupling. This makes it decidedly atypical in the history of cinema.

Think of movie threesomes and you might picture Denise Richards, Matt Dillon and Neve Campbell writhing around in a swimming pool in Wild Things. Such erotic encounters in mainstream movies spice things up, but they’re a side attraction. In comedies, they are played for laughs: Russell Brand, Jonah Hill and Elisabeth Moss had a clumsy romp in Get Him to the Greek, which also served a common dramatic purpose: to reinforce the relationship between a heterosexual couple, rather than enhance it. As Meg-John Barker, author of Rewriting the Rules, a guide to the changing nature of modern relationships, puts it: “A person being in love with two people at once is a staple of much drama, from romcoms and soap operas to advice columns and tabloid news headlines. Almost always, they are forced to choose one person and to let go of the other.”

Robinson, Professor Marston’s writer-director, says: “Poly relationships or ‘kink’ on film have usually been portrayed as salacious or transgressive in a negative way and I didn’t want to do that. Narratively, I wanted to make a really accessible story that told the story of three people falling in love. I would rotate the point-of-view of the film through each of their viewpoints. I wanted the audience to root for them to be together.”

There are, of course, other films that have taken a less judgmental approach to polyamory. The buoyant British comedy-drama Rita, Sue and Bob Too saw two teenaged girls on a council estate sharing the same man, although their relationship with each other was platonic, unlike the bisexual Elizabeth Marston and Olive Byrne. Henry & June documented Henry and June Miller’s relationship with Anaïs Nin. The Dreamers, starring Eva Green, Michael Pitt and Louis Garrel, was an arty erotic drama about a love triangle, but a troubled and incestuous one (two of the trio were siblings). The 1994 comedy-drama Threesome with Lara Flynn Boyle, Josh Charles and Stephen Baldwin was inspired by director Andrew Fleming’s own experiences. Oliver Stone’s Savages, which cast Blake Lively as the girlfriend of pot dealers Taylor Kitsch and Aaron Taylor-Johnson, may have shown the three living together in bliss, but things ended badly – as they have done in everything from the 1962 film Jules et Jim to the recent erotic French film Love.

“Sometimes open relationships are represented but they end in tragedy or difficulty, like in The Ice Storm or Vicky Cristina Barcelona,” says Barker. “There are a few more positive depictions of open non-monogamy in films like Shortbus, Kinsey, Summer Lovers, or – kind of – Her.”

The 2006 film Shortbus was certainly one of the more cheerfully liberal depictions of polyamory in film; colourfully detailing a group of New Yorkers exploring multiple partners through sex salons. But, just as many films aimed more specifically at the gay market have been, it was a niche arthouse movie, preaching to the converted. Professor Marston plays it straight enough to reach a more conservative crowd, indicating that polyamory might be going more mainstream. And the chances are the subject will crop up again in Chanya Button’s upcoming Vita & Virginia, the story of Virginia Woolf (Elizabeth Debicki), and aristocrat Vita Sackville-West (Gemma Arterton), who had an open relationship with her husband, Harold Nicolson.

Experts feel this may represent a real-life shift towards greater acceptance. “Things are changing slowly,” says Barker. “When I started studying this area 15 years ago, virtually all the reporting around polyamory was sensationalist and negative, saying it could never work, or it was ‘taking all the fun out of affairs’. Now we have a wealth of research on just how common polyamory is (about 5% of people in the US are openly non-monogamous), and about how positive polyamorous families can be for children.”

Of course, there is still no legal recognition of polyamorous relationships, and Barker points out that during the same-sex marriage campaigns, both sides argued against extending marriage rights to more than two people. But Janet Bennion, a professor of anthropology at Lyndon State College, Vermont, thinks that this, too, might change. “Currently, we are stuck in a stalemate between hypermasculine, intolerant policies designed to restore the 1950s model of traditional marriage, and a growing progressive community that yearns for sexual freedom. This latter body is gaining ground in areas like the [San Francisco] Bay Area and much of Europe, such as Berlin.” No doubt Professor Marston and his Wonder Women would approve of that.

• Professor Marston and the Wonder Women is in cinemas now