Ask any straight woman how many of her female friends she's kissed and the answer is likely to be “one or two” if not “all of them.” Ask any straight guy how many of his dude buddies he's kissed and the answer is probably going to be, “Are you out of your mind?”

Katy Perry can sing “I kissed a girl and I liked it” without throwing her sexual identity into question. But as shown by the new movie Humpday , about two schlubby straight friends in their 30s who decide to sleep together for an art project, the same isn't true for men.

“There's no question female sexuality is much more malleable,” says Roy Baumeister, a professor of psychology at Florida State University. “The evidence for why is not nearly as clear cut.”

While women kiss other women for curiosity or fun, with men, well, not so much. Why don't boys experiment?

Experts remain divided over whether we should be pointing to nature or nurture. But there is a growing body of research suggesting men and women are fundamentally different when it comes to sex, one that explains why Ms. Perry is a make-out bandit and the characters in Humpday feel the need to assure each other, “You're pretty solidly not gay.”

“Men are more either/or when it comes to their attractions to men or women,” says Richard Lippa, a professor of psychology at California State University, Fullerton, and author of Gender, Nature and Nurture . “Women seem to have more shades of grey in their attractions.”

A survey of 3,600 people conducted by Mr. Lippa in 2006, published in the journal Psychological Science, found that heterosexual women are 27 times more likely than heterosexual men to express attraction to their own sex.

Some researchers believe we can chalk this difference up to cultural influence.

“We sort of set it up that the definition of masculinity is to not be gay,” says Lisa Diamond, an associate professor of psychology and gender studies at the University of Utah. “That is not true of femininity at all.”

As a result, Dr. Diamond says, “there's a lot more cultural permission for women to experiment with other women, to talk about it, to joke about having a girl crush and stuff like that.”

In 2005, Dr. Diamond completed a 10-year study of female sexuality in which she tracked dozens of women and their relationships. About one-quarter of the women reported that their choice of sexual partners had nothing to do with gender. The study did not look at male sexuality.

Sex researchers agree that some men do experiment, although much less frequently than women.

This is not because of culture, it's biology, Dr. Baumeister says.

“It looks pretty hard-wired,” he says.

In a survey of sexuality studies published in the Psychological Bulletin in 2000, Dr. Baumeister found that social and cultural factors such as education and religion play a much stronger role in influencing a woman's sexuality compared with a man's. The relatively lower role of cultural influence on men's sexuality is reason to believe their sexual behaviour is rooted in biology, Dr. Baumeister says.

“Many sound sexuality studies have shown that women do tend to be more fluid in their sexuality than men, including their choice of sexual partners in terms of gender as well as with their sexual behaviour, ideas and attitudes,” says Heather Corinna, founder and director of Scarleteen.com, an international sexuality education and information resource for young adults.

“However, there's always some chicken and egg going on when it comes to human sexuality,” Ms. Corinna says. “While we have strong reason to believe some aspects of sexuality are pretty hard-wired, we also know many are learned.”

Living in a homophobic society may prevent boys from experimenting or, at the very least, talking publicly about experimenting. Still, differences between men and women can't all be culture, some researchers say.

In a secondary study conducted in 2007, Dr. Lippa found that results of his 2006 study were replicated in many countries around the world, including Australia, Japan, India, Malaysia, New Zealand and Singapore.

The consistency of the study's findings argues for a biological difference, not a cultural one, he says.

“Maybe in some sense women are more intrinsically bisexual,” he says. “People are beginning to more and more acknowledge – and by people I mean researchers and people who study these things – that there are some real gender differences.”

One such fundamental difference, Dr. Lippa says, is that men are more “category-specific. …

“If you're a gay man, you're turned on to men. If you're a straight man, you're turned on to women. There's a very specific category that turns you on. Women just don't seem to be category-specific in the same way,” he says.

That may explain why the two men in Humpday have such a difficult time coming to grips with doing the deed. It's a notion they can barely wrap their heads around, one they have to categorize in order to come to grips with.

As one character says of the project, “It's beyond gay.”

Don't want anything poking into me, and I like touching soft girls.

I hate this linkshell with a fiery passion.