Very enlightening interview with author Hanne Blank.
“Heterosexual” was actually coined in a letter at the same time as the word “homosexual,” [in the mid-19thcentury], by an Austro-Hungarian journalist named Károly Mária Kertbeny. He created these words as part of his response to a piece of Prussian legislation that made same-sex erotic behavior illegal, even in cases where the identical act performed by a man and a woman would be considered legal. And he was one of a couple of people who did a lot of writing and campaigning and pamphleteering to try to change legal opinion on that matter. He coined the words “heterosexual” and “homosexual” in a really very clever bid to try to equalize same-sex and different-sex. His intent was to suggest that there are these two categories in which human beings could be sexual, that they were not part of a hierarchy, that they were just two different flavors of the same thing.
via Why Heterosexuality Didn’t Really Exist Until the 19th Century | Alternet.