When a relatively inexperienced MMA fighter gets mentioned on Joe Rogan’s podcast after only two fights, that’s a pretty big deal. The UFC color commentator is one of the most well-known figures in the sport, and an endorsement from Rogan can go a long way. But for female MMA fighter Fallon Fox, a mention on “The Joe Rogan Experience” set off a storm of hate and controversy that follows her to this day.

“You can’t fight women. It’s fucking crazy,” Rogan said in a March 2013 episode of his podcast. “Get the fuck out of here. You need to fight men, period. You need to fight men because you are a man. You’re a man without a dick.”

Fallon Fox is no ordinary MMA fighter. She’s a transgender woman. And the MMA world does not know what to do with her.

Fox grew up in Ohio. She felt uncomfortable in her male body and didn’t relate to other children of the same sex. She remembers cross-dressing at age 5 or 6 and her family telling her to stop engaging in activities that society identifies with being female. Despite her family’s attempts to suppress it, Fox knew her male body was not right for her.

“The brain and body are separate, but together they’re a bracket of the human condition,” Fox said in an interview with Playboy.com. “Our minds are what determine our gender, and it was not aligned with my body from the jump. I was born a woman in a man’s body.”

As an adult Fox got married, fathered children and served in the Navy. She felt discomfort but believed that others had the same feelings and were hiding them the way she was. Eventually, through the Internet, she learned that there were others living with similar circumstances.

“I thought all male-bodied people were running, hiding and saying they didn’t want women’s bodies,” says Fox. “I started looking into it and found out there were these people called transgender people. And there’s people called transsexuals. And there’s people called transvestites. And drag queens. I realized I belonged in the transgender category, and I understood who I was.”

Even after discovering that she was transgender, Fox continued living in her male body for a little while. In 2006 she decided to undergo sexual reassignment surgery because “it got to the point where I couldn’t take not being able to walk around and be noticed or recognized as a woman. It was just depressing.”

Two years after the surgery Fallon began practicing mixed martial arts. Although Fox had only one year of high school wrestling experience when she began training, her skills were evident early and she went pro after a few amateur bouts. She won her first two professional fights with opening round knockouts. After the second bout, she came out as transgender in an interview with Outsports publicly. (She had previously told MMA officials and sports commissions.) Almost instantly the backlash began as fans protested allowing a fighter who once possessed a male body.

“She calls herself a woman, but I tend to disagree,” Rogan said on his podcast a few days after Fox came out. “She wants to fight women in the MMA, and I say no fucking way. I say if you had a dick at one point in time, you also have all the bone structure that comes with having a dick. You have bigger hands. You have bigger shoulder joints. You’re a man…I don’t care if you don’t have a dick anymore.”

Here’s the full clip from Rogan’s podcast discussing Fox:

Rogan’s publicist denied an interview request for this story.

Rogan was not the only prominent figure from the UFC to speak out against Fox. Popular fighters from the organization including Cain Velasquez, Miesha Tate and Ronda Rousey (the sport’s most popular female star) all spoke out against her. The harshest comments came from UFC heavyweight fighter Matt Mitrione. In an interview with “The MMA Hour,” he said, “That is a lying, sick, sociopathic, disgusting freak. And I mean that. Because you lied on your license to beat up women. That’s disgusting. You should be embarrassed.”

The story didn’t just gain traction in the MMA world. Cable news outlets ran coverage about Fox and the debate about her fighting women. While 2013 isn’t that long ago, it was still a different world for LGBT athletes. Jason Collins, the first openly gay NBA player, came out in an article for Sports Illustrated about two months after Fox did. And it was another two years before Caitlyn Jenner, a former gold medal decathlete, would come out as transgender and spark a national conversation about gender identity.

While many of the reactions to Fox’s coming out could hardly be considered “sensitive,” the logic behind Rogan and others’ opinions is understandable. It is unacceptable for a man to fight a woman because of a man’s natural physical advantages. But a transgender woman? That appears to be a different situation, the details and biology of which are not well-known to the public.

Fox’s coming out is covered in the documentary Game Face, which is currently on the film festival circuit. It portrays how the initial reaction to her public revelation was quick and often rooted in ignorance. The film’s director, Michiel Thomas, admits he initially questioned whether Fox should be allowed to compete.

“To be honest, I thought at first, ‘Is this fair?’” said Thomas. “I didn’t know anything about it. It is normal and important that people do their research about it because if it is unfair, then she shouldn’t fight.”

This is the question everyone is asking: Is it fair for Fallon Fox to fight women?

Fallon Fox’s coming out was not the first time the sports world addressed this topic. In 2004 the International Olympic Committee (IOC) adopted a policy on whether to allow transgender athletes. It established three requirements:

  1. The person must have had gender reassignment surgery
  2. The person have legal recognition of their assigned gender
  3. The person must have gone through at least two years of hormone therapy

The two years of hormone therapy are the most important point in the IOC’s decision. When a transgender athlete begins taking hormones the body undergoes major physical transformations. Many of the biological attributes that make men stronger than women can be undone after taking estrogen.

“There’s all types of things that changed when I took female hormones,” Fox said. “My musculature decreased dramatically. My bone density decreased dramatically. My skin got softer. The hairs on my arms got thinner. My receding hairline that was starting to go away began to come back. My body changed in more ways than I can list off.”

It’s not just personal anecdotes that back up Fox’s claim that she does not own a physical advantage over her competitors. Many medical experts support her arguments and say the transitioning process creates a complete transformation where the body becomes indistinguishable from a person born with a woman’s body.

“When a male-to-female transgender person begins taking estrogen, the proportions between their muscle and fat gets smaller,” said Dr. Sherman Leis, a renowned surgeon who is one of the leaders in transgender-related procedures in the United States. “Their muscles begin to atrophy and shrink up. Their fat increases. The proportions would be pretty close to a woman of the same size.”

Dr. Eric Vilain, a professor of human genetics, pediatrics and urology at UCLA and the co-director of the Institute for Society and Genetics, says the musculature of a male-to-female transgender person is the same as a genetic female two years after gender reassignment surgery.

“The two main things that happen after a transgender person decides to change sex is the muscle mass goes down and the bone density goes down,” said Vilain. “Research shows that muscle mass goes down significantly after the first year of hormone therapy. The IOC adopted the policy to allow them to compete if there is evidence after two years that the muscle mass is within the range of [genetic] females.”

While Vilain notes there isn’t a wealth of research on the topic, all the current studies show that it’s realistic to believe a transgender woman’s muscle mass will be within the range of a genetic female’s after two years.

Another major physical difference between men and women is the ability to produce testosterone, which helps athletes develop muscle (and therefore gain strength) and also increase endurance. Males produce around seven to eight times more of the hormone than women. Testosterone development for men comes almost entirely from the testicles. Therefore a transgender woman after surgery would not produce greater amounts of the hormone than a genetic female. In fact, since the ovaries produce some testosterone, transgender athletes could possibly have a disadvantage.

“In genetic women, testosterone half comes from the ovaries and another half from the adrenal,” said Vilain. “So if you have a transgender woman whose testicles have been removed and does not have ovaries, the only source of testosterone comes from the adrenals, so it’s lower than females.”

Neither Vilain nor Leis sees any issue with Fallon Fox, who is 5’7” and listed at 144 pounds, competing against women. Leis says that after so many years of hormone therapy Fox should “have the same muscles, hormone levels and strength as her fellow competitors.” Vilain reviewed Fox’s medical records for a story in The New York Times and saw no issues with her MMA participation.

Not every medical expert agrees. In an interview with Bloody Elbow (the SB Nation site devoted to MMA), Dr. Johnny Benjamin, an orthopedic surgeon, said there wasn’t enough research to definitively state that Fox doesn’t have a competitive advantage over her opponents. Likewise, Dr. Ramona Krutzik, a board certified endocrinologist, told Bloody Elbow that on the issue of bone density, Fox would possibly have an advantage over her opponents since estrogen actually promotes bone growth. Since men have greater bone density than women, she said, by taking estrogen Fox wouldn’t see “a great percentage of bone density loss.”

So how did Fox even qualify to become a fighter in the first place? All MMA athletes need a license from an athletic commission in order to fight. These commissions are created to prevent people with various advantages to compete professionally. When Fox got her license, she put all the facts out immediately.

“As soon as I went pro, I went to one of the highest commissions, the California State Athletic Commission (CSAC), and told them everything,” says Fox. “I told them about my gender reassignment surgery and the hormones and the process leading up to it. The medical personnel already knew about the IOC and other sports bodies that let transgender people compete. So it wasn’t hard to get the medical community on my side. And I got approved.”

The approval of medical experts and athletic commissions doesn’t mean the MMA community is satisfied. Not only have prominent figures in the sport such as Rogan and Rousey expressed their reservations, but many of Fox’s opponents have spoken out as well. Ashlee Evans-Smith fought Fox on Oct. 12, 2013, seven months after she came out, and even though she ended up winning by technical knockout, she still said she didn’t believe Fox should be allowed to compete against female competitors, and she stands by those comments today.

“She was the most powerful person I’d ever fought,” says Evans-Smith. “I’ve been rocked before, I’ve been stunned before, but never like that. Never felt like that against any female fighter. I believed I was in danger of being injured.”

Evans-Smith’s criticism might seem odd considering she beat Fox in their one fight against each other. However, she says it’s not contradictory and that MMA fights aren’t decided by strength alone.

“The stronger fighter doesn’t always win,” she says. “Numerous fights are won by the more technical fighter. We had a game plan and executed it perfectly.”

While Evans-Smith doesn’t think Fox should fight against other women until more evidence shows there is no competitive advantage, she also said that if others are willing to take her on in the octagon, that’s fine with her.

Evans-Smith’s opinions are far more respectful than another one of Fox’s opponents, Tamikka Brents (whom Fox beat in a first round TKO and who reportedly suffered a concussion and broken orbital bone in the fight). She responded to my request for an interview by saying, “I am not giving Fox anymore of my time for free. She’s an absolute asshole.”

Fallon Fox was scheduled to fight tomorrow against Randi Miller, who won a bronze medal in wrestling during the 2008 Olympics and has an MMA record of 1-0. However, Miller is part of the U.S. Army’s World Class Athlete Program, and the organization asked her to drop out of the fight, presumably so she can focus on wrestling. While there’s no indication that Miller was asked to drop out because Fox is transgender, there’s no denying that this controversy has kept some fighters from taking her on in the octagon.

It’s been over two years since Fox came out as transgender and nearly a decade since she underwent gender reassignment surgery. She’s won both of her fights since her only loss to Evans-Smith. As of now, it doesn’t seem either the UFC or Invicta FC (another professional MMA league) will be asking her to join anytime soon, meaning she’ll continue competing as part of smaller organizations such as the Prize Fighting Championship out of Denver.

Most of the medical community and athletic commissions support Fox’s desire to compete against women, as do UFC fighters Jon “Bones” Jones and Liz Carmouche. The people within the community who express reservations say they want to see more research. But on the flip side, where is the evidence that Fox has a competitive advantage? Arguments against Fox rely on the fact that men have more bone density and bigger musculature than women, both of which have been greatly reduced after years of her hormone therapy. Her body does not resemble a man’s body. Does her body resemble the average female walking down the street? No, but neither does Ronda Rousey’s. MMA fighters are physical specimens, whether male or female. She’s going against women who are trained in the most sophisticated forms of self-defense. After nearly a decade of taking hormones, her body is fully female and indistinguishable from the women she faces.

Fallon Fox is a woman, and she’s right where she belongs in the octagon.

Joseph Misulonas is an editorial assistant for Playboy.com. He can be found on Twitter at @jmisulonas.