By Alexander Kacala

The most searched term on online pornography sites this year? "Lesbian," according to Pornhub and YouPorn, two of the most-visited porn sites in the world.

The sites, which combined have approximately 40 billion yearly visits, recently released their annual reports detailing users’ viewing and search habits. They found that while "lesbian” was the top search term among women on both, among men the term was the sixth most popular on Pornhub and the 10th on YouPorn.

Women, who comprise approximately 30 percent of both sites’ viewers, were 151 percent more likely to watch “lesbian” videos, according to Pornhub's report.

“Women continue to favor lesbian porn,” Laurie Betito, the director of Pornhub’s Sexual Wellness Center, stated in the report, noting that the term was the most searched in 2017 as well. “They get to see acts they enjoy on themselves.”

As for men’s interest in “lesbian” adult content, Timaree Schmit, a sexuality researcher and host of the podcast “Sex With Timaree,” said it may involve several different factors.

“They are desiring a view of a sexual experience in which they cannot, by definition, take part,” she told NBC News. “So it has this voyeuristic, naughty, fantastical element.”

Additionally, Schmit said, some men find prefer “not have any male performers’ bodies” in the pornography they consume.

“It removes anxieties about comparing their penis size and body composition or even their own homophobic reactions,” she explained. “And for viewers who are turned off by violence and aggression by men against women, it removes the threat of that trigger."

When discussing “lesbian” scenes on mainstream pornography platforms, Schmit stressed that they “are very different in terms of content and aesthetic than the films that are made for and by queer women.”

The search term “trans” saw significant gains in 2018, according to Pornhub, which has published an annual insights report for the past six years. The site found a 167 percent year-over-year increase for the term among men and a 200 percent increase among people over the page of 45. In fact, Pornhub noted that the search term “trans” was the fifth most popular among visitors between 45 and 64. Among all search terms, “trans” came in 23rd on Pornhub and 16th on YouPorn.

When exploring trends around gay pornography and gay entertainers specifically, Pornhub found three of the top five adult entertainers searched by women were gay men. And a previous report by Pornhub found women were responsible for more than a third of the site’s gay male porn views.

“What women liked most about male-male porn is versatility, and with heterosexual porn you are going to get penetrated eventually, and that’s dull,” sexuality researcher Lucy Neville told NBC News in July after the release of her book, “Girls Who Like Boys Who Like Boys”

“A lot of ways it can play out with men is more exciting, more experimentation, more open to negotiation," she said.

Neville, who surveyed hundreds of women about their pornography-watching habits for her book, said many expressed a “strong desire to consume porn that is ethical in some way.” She added that many also found heterosexual porn “exploitative,” noting there was a general sense among those she interviewed that male actors participate in pornography because they enjoy sex, while some interviewees indicated they were worried about women performers and the gender dynamics in heterosexual porn.

Pornhub, which boasts an average of 92 million daily viewers, included a number of additional pieces of data: Stormy Daniels was the most searched porn actress; Sunday is its highest-traffic day of the week; the Philippines has the highest portion of female visitors worldwide (38 percent); and more people voted for Pornhub videos in 2018 than voted in the last presidential election.

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Brooke Sopelsa contributed.