1963, to be precise (according to Philip Larkin):
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.
One of my favourite rude song titles is ‘Cock up your Beaver’:
It appears that it’s not meant to be a double entendre at all. This is odd in itself, considering some of the filthy rhymes that Burns published alongside it.