Professional sports is one of the last bastions of the closet, with far too few athletes feeling comfortable coming out and being their authentic selves. But those that do stand on the shoulders of men and women who have come before them.

David Kopay, football

In 1975, the hunky former Redskins running stunned sports fans when he came out in his bestselling autobiography, The David Kopay Story, the first NFL player to ever do so.

"I got very, very few hate mails," Kopay, now 73, told Outsports. “Mostly the mail that poured in was amazingly supportive and telling their own stories. There were hundreds of letters forwarded to me."

But after he retired and looked for work as a football coach, Kopay found himself effectively blacklisted. Instead, he became a successful businessman in Southern California.

He continues to make meaningful contributions to the community as board member of the Gay and Lesbian Athletics Foundation and Gay Games ambassador. In 2007 Kopay announced plans to leave a million-dollar endowment to his alma mater, the University of Washington, for the school's LGBT Center.

Billie Jean King, tennis

A top-ranked women's tennis player in the 1960s and '70s, King became the first prominent professional female athlete to come out as a lesbian in 1981. She lost her endorsements, but became a role model for LGBT athletes like Martina Navratilova and Amélie Mauresmo.

"Sports teaches you character, it teaches you to play by the rules, it teaches you to know what it feels like to win and lose," she once remarked. "It teaches you about life."

King is an outspoken supporter of gender equity, helping to form the Women's Tennis Association and demand equal play for female tennis players. She is also active in the Elton John AIDS Foundation and, in 2013, was chosen to represent the United States alongside hockey player Caitlin Cahow and figure skater Brian Boitano at the 2014 Winter Olympics in Sochi, as a symbol of LGBT representation in sports.

She is among the honorees at this year's Trailblazer Honors, where she will be introduced by actress Emma Stone.

Glenn Burke, major league baseball

That this outgoing, funny and gifted athlete was gay was an open secret among his Dodgers teammates and managers. And while Burke maintained players didn't care, Dodgers general manager Al Campanis reportedly offered to pay for a lavish wedding. To which Burke responded: "To a woman?"

He also had a troubled relationship with Dodgers manager Tommy Lasorda, whose son was gay. Burke and Tommy Jr. would hang out together in West Hollywood, which made the elder Lasorda furious.

After only two seasons with the Dodgers, Burke was traded to Oakland A's, where he was underutilized and isolated. Some teammates even refused to shower with him. The final straw in his professional career came when A's manager Billy Martin stood in the dugout, looked straight at Burke and said, “I don’t want no faggot on my team.”

By the end of the 1980 season Burke had been released from his contract.

Finally free from clubhouse homophobia, Burke was at least able to come out publicly. He even competed in the first Gay Games in 1982, in track and field, and then in the 1986 Games in basketball—proof of what a well-rounded athlete he was.

Burke, who succumbed to AIDS in 1995, always maintained that prejudice against homosexuality "drove me out of baseball sooner than I should have [gone]."

Still, he took some pride in his achievements, telling People magazine that "they can't ever say now that a gay man can’t play in the majors, because I’m a gay man and I made it.”

Matthew Mitcham, diving

At the 2008 Beijing Summer Olympics, the formidable Chinese men's diving team was expected to make a clean sweep. Then came the 10-meter platform competition and 20-year-old Matthew Mitcham from the underdog Australian team pulled out a near perfect final dive—the highest single-dive score in Olympic history—to capture the gold for the Aussies.

After receiving his score and learning he'd won the gold, Mitcham leapt into the stands to hug and kiss his boyfriend.

Mitcham had come out publicly less than six months prior, and was one of only ten out gay Olympians at the 2008 Summer Olympics. His victory in the 10-mete dive was easily the highest profile win ever by an out gay man in an Olympic event.

Even though that kiss with his partner was captured by live video feed, NBC opted not to show it or even Mitcham's medal ceremony in their evening Olympics telecast. But the impact was not lost on the LGBT community: He was inundated with letters from gay teens inspired by his athleticism and openness, an experience the gold medal winner described as "extremely humbling."

Orlando Cruz, boxing

Representing Puerto Rico at the 2000 Olympic Games in Sydney, Cruz came out in 2012, the first boxer to do so while still fighting professionally.

"I have and will always be a proud Puerto Rican," said Cruz, who was among the inaugural class of inductees at the National Gay and Lesbian Sports Hall of Fame. "I have always been and always will be a proud gay man."

Renée Richards, tennis

In 1976, Renée Richards was primed to play in the U.S. Open, until organizers realized she was transgender and attempted to ban her.

Richards filed suit and, in a landmark verdict, the New York Supreme Court ruled in her favor. But despite the victory, she endured public scrutiny, the loss of family and friends, and brutal discrimination from the tennis world.

“It didn’t stop the [Women's Tennis Association] from trying to blacklist Richards her from pro tournaments until she sued, or 25 of the 32 women in the field from withdrawing from the first tournament she played,” ESPN wrote. “Crowds rooted against her. If Richards turned on a TV at the time, she could’ve found Johnny Carson and Bob Hope snickering on The Tonight Show that Richards was her own 'mixed doubles team.'"

Eventually she left tennis and returned to a successful ophthalmology practice in California, but Renée Richards' very public battle blazed a trail for future trans athletes like MMA fighter Fallon Fox and triathlete Chris Mosier.

Gareth Thomas, rugby

Rugby is one of the most physically aggressive sports in the world, so when the former captain of the British Lions came out as gay in 2009—the first in the sport to do so—it was a big deal.

"I don’t want to be known as a gay rugby player," Thomas declared. "I am a rugby player first and foremost. I am a man. I just happen to be gay. It’s irrelevant. What I choose to do when I close the door at home has nothing to do with what I have achieved in rugby."

Thomas said his teammates were instantly supportive and only wondered why he hadn't told them earlier.

John Amaechi, basketball

Years before Jason Collins became the first active NBA player to be out on the court, Amaechi disclosed he was gay in his 2007 autobiography, Man in the Middle.

"I think a lot of people would like to imagine that there was one me and they’ve got to erase all the pictures of me from before because now it’s a different me," Amaechi told us at the time. "No. [I am the] same person, just more. I was a role model before. I will endeavor to be an excellent role model from now on."

Chris Mosier, triathlon

Thanks to the efforts of Mosier and other athletes and advocates, the International Olympic Committee updated its guidelines before the 2016 Rio Summer Games to address the concerns of trans competitors.

The IOC clarified that there are no requirements for trans men to compete in the Games, and trans women do not need to undergo gender-conformation surgery in order to compete in women’s events. (They do, however, need to be on hormones for at least a year.)

"I don’t feel pressure to be a role model, but it is something I think about frequently," said Mosier, a three-time Ironman triathlete and the first trans person to compete in the grueling World Duathlete Championship.

"I try to always conduct myself in accordance with my values, which means being kind and helpful to others and doing the right thing even if it’s not the easiest thing. I often tell others, “You are who you are when no one is looking,” and I believe that. So I’m going to try my best to make good decisions at all times."

Paul Bloom, a Team USA triathlete for more than a decade, says he's "proud" to call Mosier a teammate. "I am very sympathetic to the challenges faced by transgender individuals... I applaud what Chris is doing and hope he helps to create a climate where transgender athletes are allowed to compete and excel everywhere."

Dan Avery and Josh Young contributed to this article.