Peter Thiel, the Silicon Valley billionaire who made news this summer for endorsing Donald Trump at the Republican convention, is a man who has sex with other men. But is he gay?

That question might seem narrow, but it is actually raises a broad and crucial distinction we must make in our notions of sexuality, identity, and community.

Since the Paleolithic Age, people have had sex with people of the same gender. But the notion that this made someone “homosexual” or “gay” was a relatively recent phenomenon in human history.

Beginning in the late 19th century, doctors, sexologists, and others began to argue that same-sex sex created the category of sexual orientation. Prior to that, sexual activities between people of the same gender were often considered sinful or criminal, but they rarely constituted what some in the medical community began to define at end of the 1800s as a “third sex” or “intersex” or even “homosexual.” The creation of the category of sexual orientation not only classified homosexual sex but it also led to the invention of heterosexuality.

In the United States and many parts of Europe, the development of these categories led to the broader cultural understanding that these sexual acts created identities. ...