You could be forgiven for thinking that our recent social, political, and legal fascination with the definition of the word “woman” was due to a male-bodied athlete, Lia Thomas, smashing women’s swimming records in the Ivy League and NCAA championships. But you’d be wrong.

The debate about sex differences has been simmering for decades now, between those who believe men and women are interchangeable and those like me who understand that they are not.

The former camp has long argued that women cannot reach full equality with men until all legal and social distinctions between the sexes are obliterated. Those in the latter camp know that these distinctions accord with scientific realities and are necessary for both men and women to thrive. That’s why we are advocating a Women’s Bill of Rights, which would preserve biological sex as a distinct legal category.

In this debate about sex differences, the Left seems all too willing to use transgender people, such as Lia Thomas, as pawns. It argues that with enough medical intervention, a man can become a woman and fairly compete with women and vice versa, or argue that inclusion is more important than fairness. Neither of these arguments holds. In the end, the example of Lia Thomas does not support the argument for the abolition of sex-segregated sports (or anything else). To the contrary, the swimmer is proving the opposite point — that men and women need to compete in separate leagues, or else women are unfairly excluded.

But again, the debate is much larger than sports, and much older than Lia Thomas.

The reason the Equal Rights Amendment, passed by Congress in 1972, has failed to gain enough support to be ratified into the Constitution is because Americans understand that equality and sameness are two different things. Women can be equal with men without being the same.

To some on the Left, any disparity between the sexes is evidence of a problem in need of solving. In reality, male-female disparities are often the result of innate sex differences. The most notorious example is the statistical gap between average pay for men and women, which is largely the result of men and women’s different choices about work. But there are other gaps as well: Men commit more violent crime. Women live longer. And the list goes on.

If men and women were the same, then mothers and fathers would have the same strengths and qualities. Boys and girls would prefer the same toys. But alasthey do not. Some people may see the effort to keep men and women in distinct categories as old-fashioned, traditional, or even oppressive. But the opposite is true. Efforts to force men and women into sameness are oppressive. As societies become more advanced, men and women exert their sex-based differences even more, not less.

Yes, it’s true that for many years, women were told “no.” We were denied educational and professional opportunities on the basis of sex. This was wrong. But it’s also wrong to tell a young girl who expresses interest in stereotypically male toys or hobbies that she is not, in fact, a girl. This relies just as much on gender-based stereotypes as any antiquated view about women in the workplace.

Women are diverse. We are individuals. But we all have one thing in common: our sex. And our biological sex has implications for our lives and our laws. Federal and state laws have long prohibited sex-based discrimination. Efforts to abolish sex differences will only set us back.

The Women’s Bill of Rights is the answer, and deserves wide support.

Hadley Heath Manning is director of policy at Independent Women’s Voice, the organization that, along with a coalition of women’s groups, has launched the Women’s Bill of Rights.