1923: Merriam-Webster's New International Dictionary: "heterosexuality"

"Heterosexuality" makes its debut in Merriam-Webster's New International Dictionary as as a "Med." [medical] term meaning "morbid sexual passion for one of the opposite sex."

As not-necessarily-procreative, heterosexuality was still considered abnormal or perverted according to a traditional reproductive standard.

Compare with Merriam-Webster's 1909 and Merriam-Webster's 1934. 

1  Jonathan Ned Katz, The Invention of Heterosexuality (NY: Dutton, March 1995), page 92, note 28 on page 225. Brett P. Palmer, of Merriam-Webster's Inc., Springfield,, MA, to Katz, September 17 and October 28, 1993. Mr. Palmer assures Katz that "homosexuality" and "homosexual" appears on page 1030 of the 1909 edition of Websters's International Dictionary. He also says that "heterosexuality" first appears on page xcii of the 1923  supplement of Webster's New International Dictionary, and that the contemporary defintion of "heterosexual first appears in the 1934 Second Edition of Webster's. Katz is grateful to Palmer for this information and for photocopies of these pages, now in the Katz Collection, NYPL.