1909: Merriam-Webster's New International Dictionary: "Homosexuality"

"Homosexuality" makes its debut in this authoritative dictionary as a "Med." (medical) term meaning "morbid sexual passion for one of the same sex."  Homosexuality was considered a "morbid" (diseased) passion because it was not a passion for procreating, but for sexual pleasure. 

"Heterosexuality" first appears in Merriam-Webster's fourteen years later, in 1923. See: Merriam-Webster's New International Dictionary: "heterosexuality," 1923 

And compare the 1923 entry on heterosexuality with Merriam-Webster's entry of 1934.[1]

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.