Why is it still so weird for guys like Rafael Nadal to wear hot pink?

As the No. 1 tennis player in the world, Rafael Nadal is a powerhouse on and off the court. Still, he wasn’t above so-called fashion rules when he hit the Australian Open Monday wearing a hot-pink outfit.

Top tennis pro Rafael Nadal got people talking with his hot-pink outfit. (Photo: Getty Images)

For his first-round match, against Victor Estrella Burgos of the Dominican Republic, the 31-year-old Spaniard wore a pair of hot-pink shorts with neon-pink head- and wristbands and gray shoes lined with bright pink.

The look didn’t go unnoticed on social media.

Despite its soft, innocent undertones, pink is one of the most politically charged hues on the color spectrum, used to promote marketing ideas, social agendas, and medical advancements.

“Pink was always worn freely by men and women until the early 20th century,” Valerie Steele, director and chief curator of the Museum at the Fashion Institute of Technology, tells Yahoo Lifestyle. “That’s when retailers were struck with the idea that they could sell more baby clothes if boys and girls got their own color. Until then, baby clothes were all white because they were easier to launder.”

The problem was, no one could agree how it should be marketed. For example, according to a story published in Smithsonian, a trade publication called Earnshaw’s Infants’ Department stated in 1918: “The generally accepted rule is pink for the boys and blue for the girls. The reason is that pink, being a more decided and stronger color, is more suitable for the boy, while blue, which is more delicate and dainty, is prettier for the girl.”

And department stores, even in the same city, couldn’t agree on one trend. “Best & Co. in Manhattan and Marshall Field’s in Chicago branded pink as a boy’s color,” per a story published by CNN. “Others like Macy’s in Manhattan and Wanamaker’s in Philadelphia identified pink as a girl’s color.”

Steele says the fashion world eventually came to a consensus that pink was the color of girls and blue for boys, after a millionaire publicized his purchase of two 18th-century paintings called The Blue Boy, of a boy dressed in blue, and Pinky, of a girl dressed in pink. “Pink for girls was an accidental decision, but the paintings got so much press,” says Steele.

The idea of girls wearing pink was further enforced in the 1950s, when gender stereotypes were solidified at home and out in the world, and again in the 1980s by toy manufacturers, adds Steele, curator of the upcoming exhibition “Pink: The History of a Punk, Pretty, Powerful Color.“

Still, when it comes to sports, neons are hot, in part due to their attention-grabbing effects — a theory studied by scientists at DayGlo, a company that develops technology for color enhancement.

But not everyone embraces men sporting neon pink. “The rules for what is acceptable for men is extremely narrow,” Christia Brown, a professor of developmental psychology and author of Parenting Beyond Pink and Blue, tells Yahoo Lifestyle. “Anything that is slightly feminine is rejected. In many ways, this reflects long-standing homophobia and the consistent devaluing of things associated with women. Importantly, these two biases are not unrelated to one another.”

But pink is clearly Nadal’s lucky color: During the 2017 US Open in September, he wore the bright shade all the way to victory.

Read more from Yahoo Lifestyle:

• Simone Biles reveals she was abused by Olympic doctor Larry Nassar: ‘I am not afraid to tell my story anymore’

• Danica Patrick confirms relationship with Aaron Rodgers — and promises to root for the Packers

• Matt Damon apologizes for controversial sexual harassment remarks

Follow us on Instagram, Facebook, and Twitter for nonstop inspiration delivered fresh to your feed, every day.