Cynthia Millen, an official with USA Swimming, a national governing body for competitive swimming in the United States, resigned on December 17 in protest of the University of Pennsylvania's decision to allow transgender athlete Lia Thomas to compete on the school's women's swimming team after the trans athlete competed as a male on the men's team.

The veteran official said she believed that biologically born males should not be allowed to compete in women's swimming meets.

"I told my fellow officials that I can no longer participate in a sport which allows biological men to compete against women," Millen wrote in her resignation letter, as reported by the Christian Post. "Everything fair about swimming is being destroyed."

Millen, who served as an official with USA Swimming for 30 years, declared during an appearance on Monday's "Tucker Carlson Tonight" that "The fact is that swimming is a sport in which bodies compete against bodies. Identities do not compete against identities. And from the very beginning, when you start out as an age grouper, swimmers are divided by sex and by age group."

Millen added that USA Swimming "recognizes that boys swim differently than girls" and that the differences in boys' and girls' bodies are accentuated once they undergo puberty. The USA Swimming official argued, "While Lia Thomas is a child of God, he is a biological male who is competing against women. And no matter how much testosterone drugs he takes, he will always be a biological male and have this advantage."

Thomas made headlines earlier this month when the biological male athlete competed in UPenn's male swim team and then transitioned to compete in the school's women's swimming team. The trans athlete then went on to break several women's swimming records, which the female-born UPenn swimmers believed was unfair.

In conversation with Fox News, Millen said that the message that this situation sends to women and girls is that "you do not matter, what you do is not important, and little girls are going to be thrown under the bus by all of this."

Millen's very public resignation in protest of Thomas' inclusion in UPenn's women's swim team was applauded by Beth Stelzer, the founder of the grassroots group Save Women's Sports. Stelzer has long worked hard to lobby state legislature to pass laws that require athletes to compete based on their biological sex alone.

Stelzer said she was confident that other people would speak out against Thomas and the inclusion of biological males in female sports. Thomas' female-born teammates recently spoke out after they were told to keep quiet about the issue. Some parents of female-born athletes on the UPenn women's swim team also called out the NCAA and asked them to change their rules.

More recently, Olympic gold medal swimmer Nancy Hogshead-Makar spoke out against the inclusion of male-born transgender athletes in women's teams, saying that "there is nothing fair" about Thomas competing against women on the swim team. The Olympic champion argued that trans women should be able to compete against women only if they can show that they no longer have their "sex-linked, male-puberty advantage," which she said Thomas still has.

"The rules should follow the evidence, and in this case it is clear; Thomas should not be in head-to-head competition with biological females," Hogshead-Makar concluded.