Over the past few years, futurists have been saying sex robots are on the horizon—and, in fact, they already exist. News has emerged about sex robots like Roxxxy, who can talk about current events, and Harmony AI, who responds to questions about her sexual preferences (even if it's just with "I'm not that kind of girl"). But one development has been noticeably missing: male sex robots.

RealDoll, one of the most well-known manufacturers of sex robots, has only one page dedicated to male RealDolls, compared to 33 pages of female dolls. TrueCompanion, the creator of the aforementioned Roxxxy, actually does offer a male counterpart named Rocky, but we rarely hear about him. Instead, the company's homepage gives all the attention to Roxxxy. She's also the one brought to expos and made the subject of articles.

In pop culture, too, male sex robots are the anomaly. Aside from Jude Law's charming portrayal of Gigolo Joe in A.I., the most well-known fictional sex robots are ones like the Stepford Wives, the fembots of Austin Powers, and Blade Runner's Pris. They're not just female but also stereotypically feminine. They're conventionally beautiful, sexually available, and programmed to serve men. They speak only when they're spoken to, and they don't demand anything from their users. Why is this?

The most obvious answer is that most people making sex robots—and robots in general—are straight men. "In product development, especially before large adoption in market, it's easiest to design something to solve a problem you understand," says creative technologist Gracie Page. Douglas Hines, founder of TrueCompanion, admits that he made his first robot female partly to meet his own needs. "The decision to have the world’s first sex robot, Roxxxy, be a female might have been an unexpressed desire of mine to connect with women more," he says.

But others say there simply isn't much of a demand for male sex robots: Like the companies creating them, the market consists mainly of straight men. A 2016 Tufts University study found that more than two thirds of men would use a sex robot, but only about one third of women would. Hines confirms that his customers are "by far mostly male."

"Women’s desire is more often promoted by the mind, memory, or emotional connection rather than pure physical or visual attraction," says Alicia Sinclair, sex educator and founder and CEO of b-Vibe and Le Wand. "Men are much more driven to physical objects that visually stimulate them."

Some might chalk this up to innate gender differences, but there could be another, more insidious reason some men would prefer sexual partners who are just there for their gratification—as opposed to ones who are actual humans. Because men are taught from a young age to objectify women, perhaps having sex with a literal object isn't as much of a leap for them. Men might seek in sex robots the same qualities they've been conditioned to seek in women: submissiveness, agreeableness, and passivity.

We tend to assign technology a gender based on our assumptions about humans of that gender. This was first pointed out in discussions of our disproportionately female virtual assistants (think Siri and Alexa). Kathleen Richardson, author of An Anthropology of Robots and AI: Annihilation Anxiety and Machines, told Live Science this disparity "probably reflects what some men think about women—that they're not fully human beings."

So perhaps it's the very stereotypes of femininity characterizing pop culture robots—like the Stepford Wives—that have made sex robots female by default. After all, it's these qualities that their users often cite when they describe their appeal. In the documentary My Sex Robot, one male user explains that "it's the whole idea of a man taking charge; it's almost as close to human slavery as you can get." Another says he likes "the idea of the fantasy of having something beautiful that you can control that's not gonna leave."