A 5-0 record is Fallon Fox's biggest liability right now, bigger even than prejudice. She has fought five MMA bouts and won all of them in the first round, the most recent in 39 seconds.

She won't be the first woman to be called a man because she dominated a sport. She won't even be known as the first who lived as a man for many years before becoming a prominent athlete. Renee Richards was the pioneer on that turf.

But Richards surfaced in her 40s, too late to have a shot at owning women's tennis, a sport for the young. She never ranked higher than 20th in singles.

When Fox told Sports Illustrated this week that she had undergone sexual reassignment surgery in 2006, the story went into compelling detail about the effects of her hormone therapy on the athletic advantages she would have enjoyed in her life as a man. The treatment, various experts say, suppresses her testosterone production to a level lower not only below that of almost any man, but also beneath the average for a woman. This is why, as Fox pointed out, the International Olympic Committee declared women like her eligible in 2004.

Still, plenty of reasonable people -- the vital middle ground between lost-cause bigots and completely undeterred fans -- will question the fairness of Fox fighting other women. They may not oppose it, with an ugly certainty that biology resolves gender issues at birth, but they will question.

These people may fully believe that nature can fumble the most basic identity issues and that Fox had every right to take possession of her sexuality in her own way. These people will probably admire Fox's relationship with her teenage daughter, whom she fathered in her life as a man. They would never consider calling her anything but "her'' or -- in the ultimate phobia test -- think twice about sharing a women's locker room with her.

But when they view Fox as an athlete, they might be found guilty of accepting what academics call the gender binary. In other words, they subscribe to a strict division of the two sexes, rather than acknowledging a spectrum of gender identity.

The problem with dismissing the gender binary in sport is that it's the very thing that makes women's competition possible. This whole debate is a clash of two progressive values -- fluid definitions of sexual identity and the Title IX revolution. Every female athlete in the world has benefited from a form of discrimination and segregation. Without strict division, men would claim virtually every prize in sports. They did so for ages, before women could compete.

So when advocates for transgender athletes compare their plight to that of pre-Jackie Robinson baseball players, they've gone out of bounds. Racial distinctions limited black athletes. Gender distinctions empower most female athletes.

Sports fans know that instinctively. Academics and advocates help no one by ignoring this central fact.

Already, the conversation about Fox has taken smarter turns than the one about Caster Semenya, the South African runner who had to undergo sex testing in 2009 because her rapid improvement led some people to believe she was intersex -- with a combination of female and male secondary sexual characteristics.

The whole Semenya case remains a tribute to the murkiness of enforcing segregation by sex in sports. There was no discussion of acceptable hormone levels or how her body worked in comparison to a man's, because there was no acknowledgment of her circumstances. The results of the tests were never announced formally or discussed by the athlete or her family. She returned to running 11 months later, cleared for competition in women's events without explanation.

Seeking compassion toward the teenager, academics did extensive finger-wagging about ignorance of the intersex phenomenon, while largely ignoring the consequences of such inclusiveness for other female athletes. They found the sex testing so appalling that they failed to consider the incentives an unscrupulous coach or parent might have to alter a young athlete's sexuality through unnatural means.

East German sports authorities infamously dosed a shot-putter named Heidi Krieger with so many hormones that she later underwent a sex change operation. Now known as Andreas Krieger, he has said he always felt confused about his sexual identity, but still resents his coaches for chemically pressuring him in one direction.

Semenya competed in the 800 meters in London and won the Olympic silver medal, triggering speculation that she had coasted into the finish so she wouldn't be the champ and have to endure more speculation. (The same whispering could happen to Fox when she takes her first defeat. She took a dive to validate her femininity.)

Still, the fact that Semenya did not crush the field suggests that, even if she does have some male traits, she does not enjoy the overwhelming advantage that masculinity is presumed to provide.

Likewise, Dora Ratjen, a German high-jumper later proven to be male, finished only fourth among the women at the 1936 Olympics.

So what does Fox's 5-0 featherweight record really mean? Does it say anything more than the perfect record of Ronda Rousey, the bantamweight champion, who has 10 victories, seven as a pro, all in the first round, two of them within 25 seconds? Probably not.

But if she keeps winning with ease, perception will trump reason. It usually does. Consider Richards' take on the IOC's 2004 move to include transgender athletes just like herself. She said that the decision undermined the goal of a level playing field. Sexual reassignment, she argued, manipulated the body and its achievements much like doping or corked bats.

In an interview with Selena Roberts of the New York Times, Richards distinguished the IOC conclusion from the 1977 court ruling that allowed her to compete on the women's tennis tour:

"It was an individual decision based on me,'' she recalled. "The judge was saying that I was 41, old enough to be the mother of the women I was competing with.''

 If Richards had been 25? "It would have been totally different," she said.

The truth is, neither Richards nor the judge knew what she could have done in her prime. She was 6-foot-2, and that certainly would have helped. Fox is 5-6 and has to stay under 145 pounds to compete within her weight class. Her size, no gift from her life as a man, is just one of her answers to skeptics.

She's already represented herself well in the SI interview. She'll have many more answers. But will losing be the only definitive one?