Experts say more sexual experimentation occurs when people have not yet found a partner, before they settle into monogamous relationships. College is also a safe cocoon for self-discovery.
Many scientific studies have also confirmed that both heterosexual and lesbian women tend to become sexually aroused by both male and female erotica or have a bisexual arousal pattern.
A 2003 study from Northwestern University found that compared with men, women's sexual arousal patterns may be less tightly connected to their sexual orientation and more "flexible."
Some girls say this has nothing to do with their sexual identity. They just like to live up to the ménage-a-trois fantasy of the men they seek to please.
"I think girls kiss girls to draw attention from guys, who think it's sexy and seductive," said Lauren DeGiorgi, a senior majoring in psychology at East Carolina University. "But we're usually drunk when it happens."
"Current generations are more open-minded and may promote that lax attitude by making this behavior more acceptable - probably among younger, easily influenced girls," said Heather, a college junior from San Diego, Calif., who did not want to use her last name.
And some think it even "indirectly mocks" the gay community, according to Caleb Fox, a 20-year-old from the University of Texas.
"Straight men really like the idea of two 'hot' girls making out," he said. "And because I don't think these 'flexisexuals' are really lesbians, it doesn't seem that they're actually seeking a romantic relationship with another woman -- it's more about a show. And from a feminist standpoint it continues the objectification of women."
Lisa Diamond, associate professor of gender studies at University of Utah, has been studying the topic for years, and confirms women are, indeed, more flexible in their sexuality and for a variety of cultural, and perhaps biological, reasons.
"I think there is a growing awareness of the fact that you don't have to be 100 percent gay to have the capacity to enjoy same-sex contact," said Diamond, who is author of the 2008 book, "Sexual Fluidity: Understanding Women's Love and Desire."
"In the old days, any instance of same-sex attraction was automatically put in the category of bisexual or lesbian and now we realize women are more complicated than that," she said. "There are more examples floating around in popular culture, and the term reflects that."
Claire, the sister in the HBO series, "Six Feet Under," had a sexual dalliance with a female friend, but decided it wasn't her thing and returned to boys.
Diamond said women's capacity for fluidity has always existed, but only now has society had a cultural understanding after collecting data from around the world.
"We had a pretty rigid view of the way sexuality and orientation works," she said. "But sex researchers have been aware of it for many years. It has taken folks awhile to realize you can have a periodic attraction, but that does not make that sexual identity legitimate."
Men appear to be more "rigid" in their sexuality but that may be because society is more judgmental, according to Diamond.
"Although we find it titillating when girls kiss girls and see images of same-sex sexuality being marketed to a heterosexual male audience, it's not viewed as threatening and alarming," she said. "We don't see the same cultural permission for men."
If HBO had a narrative with a heterosexual man having an encounter with another man, "no one would believe it," she said. "You never see that kind of a plot for a male character."
ABC News reporters Brianna Gays, Ashley Jennings and Tia Castenada contributed to this report.