SPOILER WARNING: This story discusses details of a single scene in Marvel Studios’ “Eternals,” which opens in theaters on Friday.

“Eternals” marks several milestones for Marvel Studios. It’s the first title in the MCU to be directed by an Oscar winner and a woman of color, Chloé Zhao. It’s the first feature film for the studio with title superheroes who are South Asian (Kumail Nanjiani’s Kingo), Latina (Salma Hayek’s Ajak), deaf (Lauren Ridloff’s Makkari) and LGBTQ (Brian Tyree Henry’s Phastos). It’s the first to feature a same-sex kiss, between Phastos and his husband, Ben (Haaz Sleiman).

And it’s the first Marvel Studios movie to feature two characters having sex on screen.

When the latter was pointed out to Zhao in an interview in late October, the director broke into a wide grin. “We win!” she said.

While “Eternals” is an ensemble film, its story is built around a central couple: Sersi (Gemma Chan) and Ikaris (Richard Madden), two ageless superheroes who fall in love over 3,000 years ago during the Babylonian Empire — and then consummate that love on screen in a sun-dappled rocky landscape.

Zhao told Variety that the scene was “in the treatment” for “Eternals” that she read before she came on board to direct the movie.

“You can’t tell a mature love story if you’re not gonna do some kind of…” she said, trailing off. “It’s lovely to see two people show physical affection for each other — a kiss, make love.”

Zhao filmed the scene between Chan and Madden on location, at the end of the day of production.

“Gemma and Richard, they did such a great job [with] the last bit of light we have,” Zhao said. “It’s very tasteful. You can feel the genuine love they have for each other.”

Sex in studio feature filmmaking has become rare, in general, over the past 20 years, with PG and PG-13 ratings dominating as storytelling skews to four-quadrant friendly franchises. Given the general appeal to kids — and a media storm caused by sexually suggestive scenes between Batman (Michael Keaton) and Catwoman (Michelle Pfeiffer) in 1992’s “Batman Returns” — sex has been especially verboten in superhero cinema, sequestered to the territory of suggestion and off-camera shenanigans. In the MCU, there’s been an (unseen) one-night stand between Tony and journalist Christine Everhart (Leslie Bibb) in 2008’s “Iron Man”; a scene set at an interstellar brothel in 2017’s “Guardians of the Galaxy Vol. 2”; and some sitcom-y, under-the-sheets whoopee on the 2021 Disney Plus series “WandaVision.” Most MCU couples either remain, at most, PG — Steve Rogers and Peggy Carter, T’Challa and Nakia — or have an outright fear of sexuality written into their character arcs to keep them apart, like Bruce Banner and Natasha Romanoff.

And that’s pretty much it. Aside from the pegging scene in 2016’s “Deadpool,” even the recent wave of R-rated superhero movies have earned their rating from graphic violence, coarse language and occasional nudity, rather than scenes of sex.

Marvel Studios is, of course, owned by Disney, which has fiercely protected its multibillion-dollar family-friendly brand for decades. And yet Zhao says she got zero pushback from the studio about the sex scene between Sersi and Ikaris in “Eternals.”

“When we put it together, we showed people at Disney, and they really loved it,” she said. “It was beautiful. And there was no discussion of taking it out.”

With superheroes continuing to dominate mass culture for the foreseeable future, it’s unclear whether the sex scene in “Eternals” is the exception to the genre’s vow of chastity, or a harbinger of a more liberated approach to sexuality on screen. One possible hint at what’s to come: In 2022, Marvel will release the fourth “Thor” movie with this eyebrow-raising subtitle: “Love and Thunder.”