1934: Merriam-Webster's New International Dictionary: "heterosexuality" & "homosexuality"

In 1934 "heterosexuality" appears in Webster's hefty Second Edition Unabridged, defined in what in 2015 is still the dominant modern mode. In 1934 "heterosexuality" is a "manifistation of sexual passion for one of the opposite sex; normal sexuality." Heterosexuality has finally attained the status of norm. 

In the same 1934 Webster's, "homosexuality" has changed as well. It's simply "eroticism for one of the same sex." Both terms medical origins are no longer cited. Heterosexuality and homosexuality have settled into standard English.

Jonathan Ned Katz, The Invention of Heterosexuality (NY: Penguin Group/Dutton, 1995), page 92.