Buckets are the modern day humans aid in understanding other humans. We often like to categorize before deciding to get to know another even more. We use buckets as filters to sort out the ones we'd like to engage in a conversation with, share intimacy with, engage in sex or even partner up for life.
Having established on this site that Marriage was a cultural phenomenon, set forth by society with different rules and expectations of different individuals, let us travel back to a time before any buckets came into being: when individuals solely acted upon their feelings and attractions as opposed to the gendered roles they had to play and sexuality, they embraced.
People would have sex with anyone they'd find attractive, irrespective of gender, age or sexuality.
Chicago Medical Journal
That was the year Heterosexual debuted in front of the world of academics. Credited to the American Psychologist, Dr. James G. Kiernan, he defined Heterosexuality as a mental condition. Combining the Greek word, heteros and the Latin, sexualis, Heterosexuality (a different sexuality that was normal for the time) was associated with Perversion.
What was the norm for that time was reproductive sex, anything beyond the norm was linked to Heterosexuality. Kiernen had commented upon the abnormal manifestations of the sexual appetite in a list of Sexual Perversions Proper.
Around that time another terminology was circulating in the mix: Psychical Hermaphroditism. Early "mind doctors" used to believe that feelings too had a biological sex.
The hetero in Heterosexual doesn't refer to one's interest in a different sex (as an identity) but one's desire for two different sexes (as an activity). Heterosexuals in short were guilty of reproductive deviance. They relied on abnormal methods of gratification, where in pleasure was guaranteed without contributing to the furthering of the species. They also portrayed traces of the normal sexual appetite, aka "a touch of desire to reproduce."
Unlike the term Homosexual, which Kiernan defined as the general mental state is that of the opposite sex, the Heterosexual, was concluded to have deviated from gender, erotic and procreative norms. To further summarize, according to Kiernan's essay, being a heterosexual was absolutely abnormal, unequivocally a Pervert.
Psychopathia Sexualis, with Especial Reference to Contrary Sexual Instinct: A Medico-Legal Study
Richard von Krafft-Ebing, Professor of Psychiatry and Neurology at University of Vienna began defining heterosexuality as engaging in sick sex. He implicitly specifies erotic normality, such as engaging in reproductive sex, while heterosexual takes the form of the contrary sexual instinct, in essence a different sex eroticism.
Towards the end of the 19th Century, Heterosexuality became what we know as today. Psychologists like Freud an influential role in stabilizing, publicizing, and normalizing the new heterosexual ideal.
Johnson, J (1973), "Psychopathia Sexualis.", The Manchester Medical Gazette (published December 1973), vol. 53, no. 2, pp. 32–4, PMID4596802
Kupferschmidt, H (1987), "Richard von Krafft-Ebing's "Psychopathia sexualis". Pornography or professional literature?", Schweiz. Rundsch. Med. Prax. (published 12 May 1987), vol. 76, no. 20, pp. 563–9, PMID3306869