The writer Karoly-Maria Kertbeny was marked at a very young age by a traumatic experience: a gay friend of his in Budapest killed himself after an extortionist threatened to expose his sexuality. Later, Kertbeny said this experience left him with "an instinctive drive to take issue with every injustice."
He began to publish pamphlets anonymously in defense of fledgling gay rights, which were under attack by the Prussian government's sodomy laws. Kertbeny also began corresponding with Karl Heinrich Ulrichs, an Austrian gay-rights activist (and also an out gay man, which, at the time, was quite courageous).
In early May of 1868, he wrote to Ulrichs with his best efforts to correctly term the varying sexualities he was able to identify at the time, and in addition to "monosexuals," Kertbeny invented the terms "heterosexuals" and "homosexuals."
Although Kertbeny never openly admitted to his own homosexuality, his private writings suggest that he may have been gay himself. Regardless, he is credited not only with coining the clinical terms we still use today for the two most common forms of human sexuality, but also with advancing other progressive sociological concepts that had yet to gain much traction on the world stage: that homosexuality may have causes grounded in science as opposed to being a choice, and that private sexual acts between consenting adults need not be criminalized in public.
To this day, the LGBT community in Budapest traditionally places a wreath at Kertbeny's grave.