Broad acceptance of gay marriage represented one of the most extraordinary shifts of public opinion on a major social issue in American history. In 1996, the Defense of Marriage Act sailed through both chambers of Congress and was signed into law by Bill Clinton. In 2004, the year eleven states passed bans on gay marriage in ballot measures, Americans opposed gay marriage by a two-to-one margin (or 60 percent to 31 percent), according to the Pew Research Center. In 2008, the same California electorate that voted for Barack Obama by 24 points also passed a measure banning gay marriage.

Yet by 2014, the year before the Supreme Court decided gay marriage was a constitutional right, a majority of Americans came to support gay marriage. By 2019, Americans supported it by 61 percent to 31 percent. In other words, during a 15-year period in American politics characterized by bitter partisan divisions, the nation went from two-to-one against gay marriage to two-to-one in favor.

Many people have operated under the assumption that the rapid shift in public opinion on gay marriage provides a preview of where the transgender-rights movement will go. Democrats have jumped on the bandwagon of referring to "pregnant people" and "people who menstruate" and many Republicans (up until recently) have been trigger-shy about engaging on the issue. Also, many of the "collapse of America" pessimists on the right have assumed that views on gender identity will follow the same path as trends on sexual orientation.

But there is a key difference between the two social-change movements. While there are many reasons for the rapid shift in opinion on gay marriage, one strong component to it was that there was a libertarian thread at the heart of it. Proponents argued that if two men fell in love, decided they wanted to spend their life together, and wanted to make it official, it should be their own business and nobody else's. Arguments made by social conservatives about the breakdown of traditional marriage did not prevail, especially with the younger generation, because people ultimately concluded that one couple's same-sex marriage poses no threat to anybody else's ability to have a happy heterosexual marriage. The libertarian argument is what helped win over a lot of small-government Republican and independent voters to the cause of gay marriage. In 2004, just 19 percent of Republicans supported gay marriage, according to Gallup, but by 2021, a 55-percent majority did.

What's substantially different about the current debate on the transgender front is that it has moved away from the successful strategy of gay-marriage proponents. While the public is broadly accepting of the idea that adults who want to identify as a different gender and undergo hormone treatment to live out their lives should be given space to do so, transgender activists are pushing for changes that have direct ramifications for others. Two men falling in love and getting married may not directly affect anybody else, but when an athlete who has gone through male puberty starts to dominate a woman's sport, it does.

Notably, with the gay-rights debate, when it came to the issue of whether bakers and photographers should be forced to provide services for gay weddings, the public was much more divided than on gay marriage, with Republicans overwhelmingly saying such businesses should be free to refuse service. Polling indicates that the transgender movement, too, is on shakier ground when it comes to gaining support for policies that depart from a "live and let live" attitude.

For instance, in a Gallup poll taken last year, two-thirds of Americans favored allowing transgender people to openly serve in the military. But the same poll found that 62 percent believed that transgender men and women should play on teams that match their own birth gender.

A YouGov poll on a variety of transgender issues found that a majority of adults said that a person should be able to legally self-identify as a different gender than their biological sex, but majorities opposed allowing transgender women to participate in women's sporting events or for biological males who have not undergone transition surgery to use women's changing rooms or bathrooms merely by identifying as women.

The women's sports issue is the one most likely to backfire on the transgender movement as more people witness what it means in practice. This can be seen dramatically in the case of University of Pennsylvania swimmer Lia Thomas, who was ranked 462 as a male swimmer but shot up to No. 1 in the nation after transitioning and being allowed to compete in the women's division.

Teammates, who have spoken anonymously due to fear of repercussions, have complained about the fundamental unfairness of allowing somebody with inherent biological advantages to compete against women who have worked hard to get where they are but now have no chance of success. In a recent interview, one teammate spoke of her "frustration" with the "insane" policy and the refusal of the school or the NCAA to support cisgender women. She recounted how teammates were uncomfortable with seeing Lia's male genitalia in the locker room but were told by the coach to just "suck it up."

Notably, the swimmer drew a distinction between the idea of being open and tolerant of transgender athletes without adopting a policy that is unfair to others.

"It is not like people are discriminating against Lia and not allowing her to swim," the Penn swimmer said. "She identified as a woman and competed on the men's team. That was the choice she was making. Then to compete with the women's team. That is something that cisgender women are not choosing. There are categories for a reason. They make sense and ensure fairness. . . . The NCAA has not said anything, and by not saying anything, they are discriminating against cisgender women."

Transgender activists will likely find a receptive public to the extent that they focus on arguing for more tolerance and compassion. But if they continue down the current path, they are going to run smack up against Americans' understanding of human biology as well as their appreciation for basic fairness.

PHILIP KLEIN is the editor of National Review Online. @philipaklein