Nate Brown and Kimberly Nelson did not expect to hear honking and yelling in the middle of shooting wedding portraits at the top of the steps of the Philadelphia Museum of Art on a balmy September afternoon in 2009.

At first, the newlyweds thought it was another wedding. But it soon became clear that this was something else entirely — a horde of naked people on bicycles, hurtling toward the Rocky statue. The couple’s photographer, Joseph Gidjunis, was the first to identify the spectacle.

>> PHOTOS: Naked bike riders bare all on Philly streets

“He was like, ‘Oh my gosh, it’s the Naked Bike Ride!’” Brown said.

Brown and Nelson were stunned. They had first heard of the Naked Bike Ride just two days before, when they arrived from Los Angeles for their wedding. Nelson’s sister, the maid of honor, had mentioned the inaugural 10-mile ride to advocate safe streets and body positivity.

“We actually said, ‘Oh, wouldn’t that be funny if it was the same day as our wedding?’” Brown said. “But that was literally it. We didn’t think anything of it.”

Gidjunis, a former photojournalist who now owns JPG Photography, asked the couple if they wanted to break from the usual poses to take some goofy, cheeky photos amongst the riders. And despite Nelson’s discomfort with nudity, the trio decided to try. They sprinted down the steps to catch the last of the cyclists speeding by in varying states of nudity.

“It was so funny because two things that make [Nelson] super-uncomfortable: PDA and nudity,” Brown said. “We were surrounded by nudity, and Joe’s making us kiss.”

The trio had about two minutes before the ride passed them by, so Gidjunis immediately got to work. He snapped dozens of photos while shouting commands — “Focus on each other! Pick her up! Kiss her!” An hour later, when Brown and Nelson arrived at Manayunk Brewery for their reception, they still couldn’t believe what had just happened.

“We have the picture hanging in our house today,” said Nelson, who now lives in Ardmore with Brown and their two sons. “It’s always really funny when people walk in and they’re like, ‘Oh! There’s a story behind that picture.’ It’s not subtle.”

Wedding photos are a must for nearly every altar-bound couple. But let’s be honest: A fresh backdrop is hard to come by. Every weekend, it seems, a bevy of brides and grooms congregate around the ornate arches of City Hall, the cobblestoned streets of Old City, or grassy knolls along the Parkway. It can feel even harder to strike an original pose when faced with a sea of kissing and hand-holding images.

So it’s not surprising that Brown and Nelson inadvertently set off a wave of Philly newlyweds snapping portraits with the legion of bare-skinned bikers who surge through Philly’s streets during the Naked Bike Ride, which takes place this Saturday. (Besides lots of flesh, you can expect to see plenty of bras, boxers, socks, helmets, backpacks, and body paint.)

Organizers of the 10-year-old event don’t know how many participants to expect, but last year’s crew was more than 1,000 cyclists strong, making Philadelphia’s iteration of the worldwide event second in size only to that of Portland, Ore. Riders don’t register in advance; they just show up the day of with a bike, water, and sunblock for their birthday suits.

Likewise, the route is kept quiet until 24 hours beforehand, when organizers send it to riders via social media. That makes photobombing tricky for adventurous newlyweds and their photographers. However, certain locales are a safe bet, including Rittenhouse Square, City Hall, the Art Museum steps, and University City.

There’s not a process for arranging photos, per se, said Maria Lily, the lead organizer of the Philly’s Naked Bike Ride. Rather, Lily, who asked to be identified by her middle name for professional reasons, said couples will often give her an idea of where they’re taking photos, and she lets them know roughly when they might get a mass mooning.

“Over the past couple of years, it’s grown in popularity,” she said. “Two years ago, we had about seven different couples reach out to us to find out what our route was going to be.”

She suspects the ride has become a photobombing magnet because it’s “very cute, very colorful, very Philly.” The “free-flowing, fun-loving” energy doesn’t hurt, either.

“When I’ve seen couples, they get cheered by the riders,” Lily said. “Being part of that ... it’s a fun memory for them.”

In 2015, Gidjunis managed to snap photos of another couple, Blair and Ross Cohen, surrounded by naked riders near City Hall.

“It was very surreal, very fast, very chaotic,” said Blair, who works as a senior recruiter at Einstein Healthcare Network. “We were in the middle of an intersection ... It was such an incredible day, but it was obviously very stressful, but when you plan so much and something like this comes out of left field, it reminds us: Just go with it.”

Philadelphia Magazine published those photos, which went viral after getting picked up by the Huffington Post and Elite Daily. The images eventually made Yahoo’s homepage and were the subject of an NPR segment.

“It was a positive vibe from everyone,” said Ross, a senior wealth strategy associate at UBS. “The next year, we came back [for the Naked Bike Ride] — Blair had made us T-shirts with our wedding pictures on them, and some of the people remembered us. They came up and high-fived us, like ‘Oh my God! It’s you again!’”

As one might expect, photographing giddy couples in a street teeming with naked riders is a far cry from the calm of Fairmount Water Works or the Penn Museum. Luckily, Gidjunis said that his background as a photojournalist prepared him for the chaos.

“It’s like you’re in a giant stadium and you’re feeling all this really positive energy, hearing hundreds of voices at the same time cheering,” he said. “I just try not to get run over.”

In 2017, Fishtown-based photographer Brae Howard used the ride as a backdrop for wedding photos near the Franklin Institute. After dashing through the museum to the street, Howard took hundreds of photos of the couple in the middle of the Parkway, to the bride’s delight — and her own.

“As a wedding photographer, it was really great to make non-traditional images,” Howard said.

The group had been on the museum’s roof beforehand, taking shots with the Philly skyline, when they spotted the unclad crowd coming toward them.

“It was so far away, we couldn’t tell what it was for a bit,” Howard said. “As it got closer, it was like, ‘Oh! That’s a lot of skin!’”