In 1948, Alfred C. Kinsey, then a little-known scientist at the University of Indiana, advanced a provocative theory about sex: a scale suggesting sexuality as a continuum, not a set of binary oppositions.

This fall Hollywood is finally catching up to Kinsey’s theories by showing us bisexuality among men, not just women. A handful of recent movies portray sex between men who are — just for the moment — wildly attracted to each other. These are characters who have relationships with women but who also have romantic or sexual relationships with other men, and it’s no big deal. There’s no fretting about their masculinity or about being labeled gay.

Fittingly, the most obvious movie to break the taboo against on-screen male bisexuality is Kinsey, the biopic about Kinsey himself, which opened Wednesday. It’s just one of several recent movies in which the man-on-man action is not awkward or played for laughs but meant to electrify audiences like classic Bogart-Bacall close-ups.

“So many young actors seem so open to it,” said Bill Condon, who wrote and directed Kinsey. “It feels like, culturally, they are willing to be the object of both men and women’s desire.”

In Kinsey, we see a moment heavy with sexual tension in which a younger man seduces an older man by asking him to make an assessment of where he lands on Kinsey’s scale. In Stage Beauty, one man is pursued first by a duke, then in the same evening by his female assistant. And in Enduring Love, the passionate homoerotic attachment at the center of the movie may be odd, but not because it’s a man obsessed by another man. It’s presented to us in a straightforward way as simply a man obsessed.

This summer brought us Colin Farrell in A Home at the End of the World, playing a character whose need for love is so great that it transcends sexual orientation. Like the stereotypical notion of women, he uses sex to get love. And on Wednesday, Farrell reappeared playing Alexander, the great man notorious for his attractions for men and women.

From Silkwood to Chasing Amy, Hollywood has been perfectly comfortable with (not to say titillated by) plot lines involving women who are not explicitly lesbians having sexual relationships with other women. But male parts have traditionally been written as either gay or straight. Only cinematic code — the lingering shot, furtive glance or double-entendre — could suggest more complex realities in men’s sex lives. Movies such as the Cole Porter biopic Night and Day from 1946 simply hinted at Porter’s bisexuality. In one scene, Porter, played by Cary Grant, is seen kissing his wife, his eyes turned to something — or somebody — else. This year, De-Lovely, starring Kevin Kline, retold the story of Porter’s life with much more than furtive glances.

The gay-rights movement brought more cinematic portraits of gay men, with the true mainstream arrival of on-screen gayness heralded by Tom Hanks’ earnest, antiseptic and prudish portrait in Philadelphia a decade ago. Hanks won an Academy Award without having to kiss Antonio Banderas. But there have been few portraits of male sexual ambiguity on screen.

In the movies out now that explore male bisexuality, men get to play the object of affection rather than the seducer, taking a role that in the past belonged to women. They are often pursued and passive in love.

What Kinsey found — a radical notion in postwar America — was that men are varied creatures with a variety of needs that often change as they pass through their lives. (A later book explored women and their sexual behavior.)

In the movie, which turns on Kinsey’s work and love life, Laura Linney plays a student who falls in love with and later marries Kinsey. In one classroom scene, she is seen gazing adoringly at her teacher. Later, Peter Sarsgaard’s character, the student Clyde Martin, looks up at his teacher during a “marriage class” — the sex education class initiated by Kinsey — in a way that makes their attraction clear.

“He’s in that kind of traditional role of the smitten student,” Condon said. “There’s a real kind of doubling up: First the heterosexual romance, then the homosexual romance.”

A model of a new kind of on-screen masculine sexuality, Sarsgaard is softer, self-assured and “so in touch with his feminine side,” said Condon. “It’s sort of what Kinsey was talking about: Peter is so comfortable in his own skin. Anything that makes up who he is, is something he is comfortable with.”

Sarsgaard belongs to a new generation of actors including Billy Crudup, Gael Garcia Bernal, Jake Gyllenhaal and Tobey Maguire who play straight perfectly well but display a more fluid sexuality, a “certain kind of sensitivity that isn’t all tied up in the Brando myth,” said Condon. “It’s not bad boys in leather jackets but a genuine kind of expression of a sensitive nature.” As these actors convey it, on-screen bisexuality isn’t shocking or transgressive but merely human.

Set in 1660s England, when women weren’t allowed on the stage and so were played by men, Stage Beauty is loosely based on the life of one such actor, Edward “Ned” Kynaston. Played by Crudup, Kynaston has an affair with the Duke of Buckingham (Ben Chaplin) and then with his assistant, Maria (Claire Danes).

“The romantic lead by definition, according to Hollywood, is heterosexual,” said Richard Eyre, who directed Stage Beauty. “[Kynaston] doesn’t conform to the stereotype or the archetype, but that’s what makes him interesting.”

Taught to portray women on stage, Kynaston gives a version of feminine behavior that is stylized, even kabuki-like. But as Crudup plays him, Kynaston is sexually attractive to women even as he performs as one.

The question of his sexuality — like Kinsey’s — doesn’t define him as forcefully as his inability to love does. This is the story of a narcissist who learns to love someone other than himself.

“Ned is interested in sex with these people because of what they offer. Emotional attention is what’s sexually arousing to Ned, not physical equipment,” Crudup said. “What is novel about the movie is that it doesn’t attempt to explain or reduce his sexuality. It just presents it as part of human experience.”

In any event, Crudup said, “descriptions like homosexual, heterosexual, metrosexual don’t do an adequate job of explaining what it’s like for people to be alive.”

Ultimately, Kynaston is not “turned straight” through the love of a good woman. His sexuality remains ambiguous. When Maria finally asks him, “Who are you now?” he answers, “I don’t know.”

But it’s P.S., another movie that opened in limited release this month (but does not yet have a South Florida release date), that may raise the most unsettling question posed by our new ability to look squarely at male bisexuality: What about the repercussions for women? Linney plays Louise, an admissions officer at Columbia University in her late 30s who reluctantly embarks on an affair with a younger artist, played by Topher Grace.

When her ex-husband, Peter, played by Gabriel Byrne, confesses that he’s a sex addict who’s had hundreds of affairs during their marriage, he also lets slip that some were men. “How many?” she demands. About a dozen, he confesses. She breaks down, sobbing. Then she says, in a voice more anguished than confused, “You’ve slept with more men than I have.”