Tennis legend Martina Navratilova was criticized last year after calling for standards to be set as an increasing number of transgender biologically-male athletes are competing in women's sports.

After promising to research the topic further, she penned an op-ed on Sunday reaffirming her views and calling it "insane and cheating" to allow a man to switch genders in order to "win everything in sight."

What are the details?

In December, Navratilova tweeted, "You can't just proclaim yourself female and be able to compete against women. There must be some standards, and having a penis and competing as a woman would not fit that standard."

Online outrage ensued. The nine-time Wimbledon champion — who in 1981 became one of the first prominent worldwide athletes to come out as bisexual (and then subsequently as lesbian), and who has long campaigned for LGBT rights — was labeled "transphobic" in a series of responses by transgender cyclist Rachel McKinnon. McKinnon was born male and won an event at the UCI Masters Track World Championship in the women's division last year.

In response, Navratilova deleted her original message, and promised to educate herself on the matter before writing, "Rachel, you might be an expert on all things trans but you are one nasty human being," adding she wouldn't be "bullied" into silence.

But McKinnon persisted in her tirade against the tennis star, telling Navratilova, "You are not 'pro-trans people' if you say that trans women with a penis must not compete in women's sport. That's transphobic. Genitals do not play sports. What part of a penis is related to tennis? How does that 'level' any playing field?"

On Sunday, Navratilova penned an op-ed in The London Times, where she reaffirmed her position that "the rules on trans athletes reward cheats and punish the innocent." Navratilova said that after further researching the issue, her views on the topic were even more strengthened.

"To put the argument at its most basic: a man can decide to be female, take hormones if required by whatever sporting organization is concerned, win everything in sight and perhaps earn a small fortune, and then reverse his decision and go back to making babies if he so desires," Navratilova wrote. "It's insane and it's cheating. I am happy to address a transgender woman in whatever form she prefers, but I would not be happy to compete against her. It would not be fair."

She further argued, "Simply reducing hormone levels — the prescription most sports have adopted — does not solve the problem. A man builds up muscle and bone density, as well as a greater number of oxygen-carrying red blood cells, from childhood."

Anything else?

Navratilova's piece attracted both praise and ridicule, with critics and supporters alike pointing to the fact that Navratilova played and beat trans female tennis player Renee Richards in 1977. Navratilova — a lesbian and avowed feminist — eventually hired Richards as a coach.

Richards was born male and began competing in women's tennis at age 40. Reflecting back on her decision, a 77-year-old Richards told Slate in 2012, "Having lived for the past 30 years, I know if I'd had surgery at the age of 22, and then at 24 went on the tour, no genetic woman in the world would have been able to come close to me. And so I've reconsidered my opinion."