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One reason the new Hulu series Casual is so entertaining to watch is that it’s so realistic. The show follows the lives of a newly divorced mom, her teenage daughter, and an uncle who created a Match.com-esque dating site—and it offers an almost-harrowing glimpse at the trials of online dating. For those who engage in the ritual, watching Casual can be like watching your own fumblings acted out on screen. It's a little terrifying.

But at the beginning of the most recent episode, the third in the series, we got a look at something most of us aren’t so familiar with: the inside of a high school sex education classroom.

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While the show’s portrayal of the class is obviously fictional, the minute we spend inside teen daughter Laura’s (Tara Lynne Barr) classroom is valuable because it shows us something of a platonic ideal for how a great sex ed lesson might look. The teacher, Dr. Tran (Keisuke Hoashi), seems to be giving his students real information about STIs—we enter the classroom as he finishes up a sentence about how often you should get screened—and when faced with an uncomfortable question, he unflinchingly gives an honest answer.

As a joke, Laura's classmate Mia (Taylor Spreitler) asks Dr. Tran why guys are “so obsessed with giving it from behind" (a question many adults may also have). The class laughs and she smiles. It’s a great troll move. But Tran, fictional sex ed teacher of the year, doesn’t take the bait.

“Now that is a very interesting question,” Dr. Tran answers. The smile on Mia’s face starts to fade away.

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“It has been hypothesized by evolutionary biologists that the sexualization of the buttocks comes from our time as primitive quadrupeds,” he continues. “We used to have a direct line of sight to the genitalia of our prospective mates, creating an obvious preference for rear entry sex—or as you say, for 'giving it from behind.'"

"However, as our ancestors evolved into bipeds, our sexual perspective shifted away from the buttocks and onto something else: breasts. See, the animal inside of us still craves rear entry sex, but the desire to go face-to-face, or breast-to-breast, is something almost distinctly human.”

This scene is supposed to be funny. The humor here lies in the idea that this is not the kind of answer the class—or anyone watching the show—expects to hear from Dr. Tran. It would’ve been less funny if Tran had reacted as most real sex ed teachers probably would—by laughing the question off and moving on.

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But that’s not what he does. And that’s what makes the scene great. Tran's answer is so serious, so honest, the entire class is left floored. And quite honestly—I was, too.

I read about sex almost every day as part of my job, but the idea that humans may be drawn to rear entry sex (or "doggy style," as the kids say) thanks to evolution was totally new to me.

The hypothesis Tran references in Casual is not oft-cited, but it is mentioned in academic articles and books on human sexuality, including Human Sexuality: An Encyclopedia.

From a chapter titled "Love and Sex" (emphasis is ours):

Assuming that natural selection for upright bipedalism was taking place coterminously with the gradual loss of estrus [heat], sight would have largely replaced smell as the impetus to mate. Upright posture in hominids, with genitals now moved more toward the front, led to the uniquely human practice of frontal intercourse. Frontal intercourse involves far more skin contact than the old method of seizing the female from behind and staring off into space.

In other words, when people started walking on two feet instead of on all fours, our genitals were no longer on clear display for all the world to sniff, and males no longer felt the need to come at women from behind. In scientific terms, it sounds pretty crass and a little bit disturbing. But the idea as Dr. Tran presents it—that face-to-face sex is "distinctly human"—is kinda nice. Almost romantic.

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Either way, Dr. Tran deserves some sort of award. Not only did he expertly dodge the teen's trolly sex question, but he taught me (and probably lots of other people) a compelling new theory.

Hannah Smothers is a reporter for Fusion's Sex & Life section, a Texpat, and a former homecoming princess.