Back when homosexuality was still listed in psychiatry manuals as a mental disorder, Karen Hooker decided to study the mental differences between homosexuals and heterosexuals. She recruited both straight and gay people, and gave them a series of psychological tests, which showed no differences between the two groups. Based on her research, in 1973, the American Psychiatric Association removed homosexuality from its Diagnostic and Statistical Manual, and announced that homosexuality was not a mental illness.
It was an enormous victory for gay rights, and has fundamentally changed the way we talk about sexual orientation. Early libertarian arguments emphasized the right of sexual minorities to choose: “What business is it of society’s what people do in their bedrooms, as long as there’s consent?” In 2003, that privacy argument persuaded the Supreme Court to protect “homosexual persons,” saying: “The state cannot demean their existence or control their destiny by making their private sexual conduct a crime.”