I know where I'll be Sunday night. The reviews coming out of Cannes for Steven Soderbergh's Liberace biopic, Behind the Candelabra, which airs on HBO on Sunday night, have turned it into must-see TV.

We might have been able to guess that Soderbergh's take on the kitsch-addicted superstar would turn out to be "mesmeric, riskily incorrect, outrageously watchable and simply outrageous" (The Guardian). Or that Michael Douglas would be "shrewd, rude, wickedly funny" (Indiewire) in the central role. What is interesting is that the film, which was made for HBO because it was "too gay" for mainstream cinematic release, has turned out to be "both hilarious and heartrending" (The Playlist), an "intimate love story" (Thompson on Hollywood) and Soderbergh's "most emotional and touching work" to date (Hollywood Elsewhere). This is interesting but not surprising, for it only confirms my suspicion that the best love stories, these days – the most wrenching, plangent and affecting – are gay.

Put it this way: what was the last great straight love story you saw? There are many who swear by The Notebook (2004), although as Hollywood continues to drive the Nicholas Sparks bandwagon into the ground, even Rachel McAdams poked some sly fun in Woody Allen's Midnight in Paris, where she questioned the sacred Sparksian connection between romance and rain: "What's wonderful about getting wet?" What heresy! Off with her head!

Before that, you have to go back to the mid-nineties, to The English Patient (1996) and Clint Eastwood's The Bridges of Madison County (1995), although by 2011 even Clint had sensed the zeitgeist shift, jumping ship to squeeze a tear from the final reel of J Edgar, one of a small raft of films from the last decade or so – including Boys Don't Cry, Of Gods and Monsters, Brokeback Mountain, Milk, I Love You, Philip Morris, A Single Man, Weekend, and now Behind the Candelabra – which have taken as their subject the knotty, semi-submerged tussle that is maintaining a gay relationship in a straight world.

In Liberace's case, the weird lengths he went to, which included plastic surgery and adoption, were partly a function of his celebrity. "Would the marriage have ended any less disastrously, in a dry hail of paperwork and stern lawyers' tones, had it been officially sanctioned?" asked Guy Lodge.

Probably not, given Liberace's vampiric reliance on younger men as a kind of elixir. ('I've always had an eye for new and refreshing talent,' he says, in one of [writer Richard] LaGravenese's most memorable exchanges, to which [Matt Damon's] Scott's priceless snapback is: 'No, you've always had an eye for new and refreshing dick.') But as the hard-won tenderness of the film's final moments suggest, homosexuals also have the right to end their relationships as ceremoniously as they begin.

The semi-closeted nature of Liberace's love affair and the obstacles he faced in pursuing it, as well as the lengths to which he went to overcome those obstacles – all are part of the classic narrative DNA of the love story, whether Gone with the Wind or Brief Encounter, Dante or Plutarch. "Unless the course of love is hindered there is no 'romance'," wrote Denis de Rougement in his classic 1940 history of the love affair, Love in the Western World. "The spontaneous ardor of a love crowned and not thwarted is essentially of short duration. It is a flare-up doomed not to survive the effulgence of its fulfillment." Love obstructed is passion fulfilled. "This obstruction is what passion really wants – its true object."

Today, we are too liberalized for Brief Encounter. Middle-class respectability, the horrific ostracization visited upon divorcées and the war-time call of duty have all receded as impediments sufficient to thwart lovers of the ardor of Celia Johnson and Trevor Howard. However, I still bow down to Eastwood's The Bridges of Madison County, for the cunning with which it baited its trap, framing the story from the point of view of the kids on whose behalf Meryl Streep sacrificed her love. An audible honk escaped me – I am not ashamed to say it – when I first saw Meryl's hand fell from her car door handle, as Eastwood pulled away in his truck. When Clint goes mushy, hetero males weep with impunity.

Straight romance is too easy, too dramatically slack, too impediment-poor, although both Like Crazy (2009) and Richard Linklater's Before Sunrise films have made great use of geographical displacement, a problem lifted for the third film in the series, Before Midnight, which is released this week. The couple at the centre of the film, Celine (Julie Delpy) and Jessie (Ethan Hawke), are now in their forties, surrounded by all the blandishments of middle-age: kids, ex-wives. "That was the conflict this time," Linklater told me when I interviewed him last month. "That was the question: can this even be romantic? Or do we go some other way?

The bedrock of our approach here was: its going to be romantic for realistic people, which is what we think out audience is to some degree, but it's out of the bubble and into the real world. By 40, it's pretty romantic at that age if you're still talking, still making each other laugh, still want to sleep together. That's about as good as you're going to do. That's not bad.

Linklater is first to admit that the films are pre-internet, pre-email. They couldn't work in the hyper-linked age of twitter and Skype. Turn to a film like Brokeback Mountain, on the other hand, and the dramatic stakes sky-rocket: romance becomes a life-or-death issue while the necessity of keeping it secret pushes feelings many fathoms deep, where the very best screen acting likes to go fishing. There's a reason, other than Hollywood liberals' self-admiration, why Sean Penn, Colin Firth and Heath Ledger have either won or been nominated for Oscars for playing gay. It reintroduces a tragic dimension to the love story, with a view of love as an ennobling but doomed force in human affairs.

And therein, of course, lies the downside, or the danger. In 2010, reviewing the tearjerkers Eloise and Prayers for Bobby at London's Gay and Lesbian film Festival, Dee Rudebeck asked: "Why aren't there more feelgood gay films? Gay cinema can do drama, comedy, romance, sex and tragedy rather well. But it's not so hot on happy-ever-after endings. The gay character usually winds up dead, or mad. Or mad, then dead."

That's why Beyond the Candelabra is to be celebrated, for the heart it wears on its sleeve, but also the sprinkle – the armful, the bucketload – of sequins it wears there, too.