A graduate student created a Craigslist ad to research male sexuality and found a hidden type of sex straight males from rural areas are having with each other: "bud-sex." 

Tony Silva, a doctoral sociology student at the University of Oregon, interviewed straight men, including several who are married to women, who secretly have sex with other straight men.

"The participants engaged in bud-sex, a specific type of male–male sex that reinforced their rural masculinity and heterosexuality," Silva wrote in a paper published last month in Sage Journals. "The married men framed sex with men as less threatening to marriage than extramarital sex with women, helping to preserve a part of their lives that they described as central to their straightness."

The men did not consider this type of sex to be gay sex, just friendly "bud-sex" between straight men. 

Flickr/Mobilus In Mobili - flic.kr

"Identities like straight, gay, lesbian, bisexual, queer, pansexual, and others refer to a complex matrix of attractions, sexual practices, and interpretations of each," Silva told ATTN:. "It was this interpretive element that I was hoping to explore further, to see how participants constructed straightness and normative masculinity despite engaging in sexual practices that wider society frames as gay or bisexual."

Silva interviewed 19 white, straight, and self-described "masculine" men from rural areas in Missouri, Illinois, Oregon, Washington, and Idaho about their sexual lives. The men volunteered for the interviews through ads Silva placed on Craigslist. Although the sample size is too small to be considered a conclusive study, it does provide some insight on how these men think about gender and sexuality.

Here are three things the rural men engaging in "bud-sex" have in common: 

1. Masculinity is important to their identity. 

Flickr/Quinn Dombrowski - flic.kr

All of the men Silva interviewed reinforced their masculinity to distance themselves from the stereotype of gay men. 

One man, Cain, explained, "My demeanor may be more gentleman-like than . . . the rugged cowboy type," but Silva wrote that he "described himself as having 'a type A personal-ity' with the potential to 'be kind of aggressive.' Similarly, David is an 'alpha male' who enjoys shooting and fishing. Richard described himself 'as masculine as John Wayne; I’m definitely not feminine in any way, shape, or form.'"

2. They are attracted to other men who have a similar background and identity. 

Flickr/David Wilson - flic.kr

Silva wrote that an important part of "bud-sex" is for both men to identify as straight. 

"The participants overwhelmingly preferred to have sex with men like themselves: masculine, white, and not gay—straight or secretly bisexual," he wrote. "This is a key element of bud-sex." 

Some men even used gay slurs when speaking about gay or "feminine" men. 

"As Cain said, 'I’m really not drawn to what I would consider really effeminate faggot type[s],' but he does 'like the masculine looking guy who maybe is more bi," wrote Silva. "Similarly, Matt explained, 'If they’re too flamboyant they just turn me off.'"

3. Secrecy is critical. 

Flickr/David Wilson - flic.kr

Because these men live in rural areas, they saw secrecy as an important way to protect against gossip.

"They understand that they would face severe social consequences if their same-sex sexual practices were discovered, such as a loss of family, friends, careers, and reputations," Silva told ATTN: "They also know that many other people would perceive them as gay or bisexual, when in fact they identify as straight."

Cultural pressure in rural areas can play a big part in sexual identity. 

Flickr/Ian Sane - flic.kr

Silva said that the men he interviewed just want to be "normal" and  "heterosexuality is central to normative rural masculinity." Indeed, "While they understand that many others would perceive them as gay or bisexual, they see themselves as straight and do not wish to have an identity imposed on them."

RELATED: Why Gay Men Have Sex With Women and How They Explain It