In 1974 UK poet Philip Larkin published "This Be The Verse" which contained the poem Annus Mirablis, with the oft quoted first verse:
Sexual intercourse began
In nineteen sixty-three
(which was rather late for me) -
Between the end of the Chatterley ban
And the Beatles' first LP.
Putting the word Sex into Google Ngram shows how the frequency of the word in books scanned by Google, for the period 1900 to 2000 tool off in the mid-sixties.
Marriage was added as a reference point.