I read an article on the BBC whose title caught my interest. It's called "The invention of heterosexuality". It's quite a lengthy article, and goes through what it claims is the very short history of the word "heterosexual", various interpretations of early philosophers and researchers (von Krafft-Ebing, Freud, Kinsey), and many other things.
What I found most interesting were both that the word originally was defined in dictionaries as a pathology.
The 1901 Dorland’s Medical Dictionary defined heterosexuality as an “abnormal or perverted appetite toward the opposite sex.” More than two decades later, in 1923, Merriam Webster’s dictionary similarly defined it as “morbid sexual passion for one of the opposite sex.” It wasn’t until 1934 that heterosexuality was graced with the meaning we’re familiar with today: “manifestation of sexual passion for one of the opposite sex; normal sexuality.”
So I checked the Merriam-Webster Dictionary and the word results in:
1 a : of, relating to, or characterized by a tendency to direct sexual desire toward the opposite sex b : of, relating to, or involving sexual intercourse between individuals of opposite sex
and then as to its first usage:
First Known Use: 1892 in the meaning defined at sense 1a
This is in complete contradiction with the claim in the article as to the 1923 definition.
I was in fact surprised that both "heterosexual" and "homosexual" are of quite recent coinage.
From Wikipedia I got this:
The term "heterosexual" was first published in 1892 in C.G. Chaddock's translation of Krafft-Ebing's "Psychopathia Sexualis". The noun came into use from the early 1920s, but did not enter common use until the 1960s. The colloquial shortening "hetero" is attested from 1933. The abstract noun "heterosexuality" is first recorded in 1900. The word "heterosexual" was first listed in Merriam-Webster's New International Dictionary as a medical term for "morbid sexual passion for one of the opposite sex"; however, in 1934 in their Second Edition Unabridged it is defined as a "manifestation of sexual passion for one of the opposite sex; normal sexuality".
Apart from the sentence that the term didn't become popular until the 1960s, the information in Wikipedia seems to square with that in the BBC article. However caveat lector: The citation marked  is the same source as cited repetitively in the BBC article, an essay with a title identical to the BBC's article's, by Jonathan Ned Katz, published in 1990 in the Socialist Review magazine. I've read through Ned Katz's information and the BBC article writer seems to have just repeated everything written by this Katz person.
I'm not very good at hunting down dictionary editions and quotations from specific time periods, so I was wondering if anyone could shed light on this. Basically that the word "heterosexual" didn't lose its "morbid" connotation until about 1934.