With Black Swan and The Kids Are All Right vying for Oscars, it seems Hollywood is growing up ... but the best roles still go to straight women

The two favourites for the 83rd Oscar for best actress are Natalie Portman and Annette Bening and, if either of them wins, the ceremony will also mark a momentous night for many more women: it will be the night when lesbian sex scenes became part of the cultural mainstream.

Bening's role as the strong matriarchal figure in a gay family in The Kids Are All Right naturally involves showing the daily intimacies of life with her on-screen partner, played by Julianne Moore. In contrast, Portman's brittle portrayal of the prima ballerina at the centre of Black Swan, a part that has already earned her both a Golden Globe and a Bafta, draws her into a lesbian encounter with a rival ballet dancer that is far from domestic. In both these very different films the gay content is presented as merely incidental to the plot. In fact, of course, it is key to what makes both screenplays feel like fresh, modern stories.

In director Darren Aronofsky's Black Swan, the sexualised rivalry between female leading characters is no longer used as a background note, as it has been in popular thrillers since the heyday of film noir, right up to Nicolas Roeg's Black Widow in 1987 or Barbet Schroeder's Single White Female in 1992. Instead, it takes centre stage. Lisa Cholodenko's The Kids Are All Right can claim to be groundbreaking, too. It is the first mainstream hit to herald an age when legalised marriage between women might be accepted across the US.

As Colin Firth demonstrated last year with his Oscar nomination for A Single Man, playing a gay character can be a rewarding challenge for a straight leading actor – and one that often brings critical plaudits.

But this year the lesbian sex scenes on screen have become more explicit and more frequent, particularly when compared to the scarcity of Hollywood sex scenes between gay men.

At the Sundance film festival in Utah this year, several new films also put lesbianism in the spotlight. Industry excitement concentrated especially on Pariah, a coming-of-age film from director Dee Rees that told the story of Alike, a gay African-American teenager in New York. The premiere met with a standing ovation and Focus Features has snapped up the worldwide rights to distribute the film ahead of interest from the Weinstein Company and Sony Pictures Classics. Another hit at the festival was a camp, comic treatment of lesbian love. Codependent Lesbian Space Alien Seeks Same tells the story of a reticent, greetings card shop worker whounwittingly falls in love with a lesbian alien called Zoinx.

Before the mid-1930s the restraints of the Hays Production Code in Hollywood meant that depictions of homosexuality were specifically forbidden. Even in the more liberated decades that followed, the physical passion of one woman for another was only ever hinted at on screen, as in the 1961 film version of Lillian Hellman's play The Children's Hour, or else confined within the walls of arthouse cinemas, as with modern gay classics such as 1985's Desert Hearts.

Axel Madsen's 231-page study of Hollywood's secret lesbian group, The Sewing Circle, caused uproar when it was published in 1996. It focused on close friendships between Greta Garbo, Marlene Dietrich, Judy Garland and Joan Crawford, and claimed that Myrna Loy, Tallulah Bankhead, Elsa Lanchester, Barbara Stanwyck and Fred Astaire's sister Adele were all involved in a thriving lesbian scene. Madsen's clear implication was that a network of covert homosexual activity still dominated Hollywood. Now many actresses, such as Ellen DeGeneres and Portia de Rossi, are openly gay.

The casting of Portman, Moore and Bening, all straight actresses, in the roles of women who are bisexual or lesbian has provoked acrimony in Hollywood's gay community. Some argued that only well-known heterosexual stars were happy to take gay parts because they could be confident their career would not suffer. This view was drily echoed by gay British novelist Stella Duffy this weekend. "It seems it is always fine for straight women to play lesbians – in fact, they quite often get Oscars for it," she said.

Casting gay women in straight female roles is more of a problem. Several well-known Hollywood leading ladies are thought to be lesbian, but have decided to keep it quiet. Just as gay actor Rupert Everett recently admitted to the Observer that he "would not advise any actor necessarily, if he was really thinking of his career, to come out", so female starlets who want a shot at the A-list must still lead a double life.

Last month lesbian actress Jane Lynch, who plays bitchy cheerleading coach Sue Sylvester in Glee, showed some sympathy for the problems facing casting directors. "This is a business of projection and desiring people from afar," she said. "And watching people go through trials and tribulations, so there has got to be some truth to it, in terms of, 'I could see myself with that person'. Because the leading man and lady are the people we want them to fall in love with, and most of the audience is straight. So, for right now, we can only use straight actors."

While the era in which Rock Hudson was forced to marry his agent's secretary to keep up appearances is behind us, Neil Giuliano, president of the Gay & Lesbian Alliance Against Defamation (Glaad), points out that it is still legal to be fired from a job in 30 of the American states for being gay and there is no federal hate-crime legislation. Coming out can still be dangerous for anyone, let alone a public figure. Duffy recalls talking to a rising British stage star who told her she had been advised to stay "in the closet".

"I was shocked to speak to a young actress who had been told by an older gay actress that she should definitely not come out if she wanted to get a range of roles. And this was an actress who had just done a film and a three-month stint in a regional theatre!" said Duffy. "I was really surprised because I knew she was gay and she was successful and she did not know whether to be out or not."

Duffy suspects the difficulty lies in the fact that there are still few high-profile lesbians in the entertainment industry. "It has been easier for women to stay invisible; not right, but easier. We didn't ever have the law against us. Men have had to come out and fight."

Several leading Hollywood actresses have come out in recent years, from Amber Heard, star of Pineapple Express, to Lindsay Lohan and Sex and the City's Cynthia Nixon, but their decision still often coincides with a tacit acceptance they may no longer be leading-lady material. Meredith Baxter, the star of the sitcom Family Ties, announced she was gay live on television, while Kelly McGillis, the leading lady in Witness and Top Gun, came out on SheWired.com, one of the largest lesbian-oriented websites in America. "I am done with the man thing," McGillis said. "I did that and need to move on in life."

The actress, who had married twice, said that coming out was a hard process that had started when she was a girl. "It was a long, arduous journey for me," McGillis said. "I had a lot of things happen that convinced me that God was punishing me because I was gay, so that was a hard process for me."

Historically, lesbian characters in film are often portrayed as threatening. Just as homosexual or effeminate men are viewed with suspicion in many screenplays, so gay women are associated with predatory obsession.

From the Beryl Reid character in The Killing of Sister George, to the malevolent and deluded teacher who is played by Judi Dench in Richard Eyre's film of Notes on a Scandal, cinema's crop-haired lesbians are clearly to be avoided. Beautiful young ballet dancers, however, can at least expect to be granted a sex scene.

This sort of unbalanced representation of gay women in mainstream show business is inevitable, Stella Duffy is convinced, as long as Hollywood is controlled by male bosses.

"The reason we see a lot of gay female sex on screen now is because straight men tend to get off on seeing gay women on screen and they don't get off on seeing gay men. And men are in charge. It is as simple as that," she said.