Mind your language
In They Came to Baghdad, a topical-sounding novel by Agatha Christie, the heroine, Victoria Jones, finds 'all was above board, mild as milk and water.... Various dark-skinned young men made tentative love to her.' Or so I am told by Mr Bruce Harkness from Kent, Ohio.
He also has, on occasion, to write footnotes explaining Conrad novels, and for Almayer's Folly he found he had to explain the following phrase: 'whether they made love under the shadows of the great trees or in the shadow of the Cathedral or on the Singapore promenade'. The problem was that readers took make love to mean 'engage in sexual intercourse'. Mr Harkness wonders until how recently writers could use the phrase in the more innocent social sense.
It is not much help looking in the Oxford English Dictionary, where under 'love' (meaning No. 8) it has 'to make love: to pay amorous attention; now more usually to copulate'. Well, speak for yourself.
The phrase 'to pay amorous attention' is lifted from the first edition of the OED, which for words beginning L-N came out in 1908. There was no mention of copulating. Yet the citations show that the amorous attentions could be close. It quotes Cowley's Hymn to Light (1663): 'Thou golden Shower of a true Jove! Who does in thee descend and Heav'n to Earth make love!' Since Jove or Zeus did to Dana‘ as a shower of gold exactly what he did to Leda as a swan, Cowley is referring to more than chit-chat.
And there is that bit in Hamlet where he is talking to his mother of her behaviour with his father's brother: 'In the rank sweat of an enseamed bed/Stew'd in corruption, honeying and making love.'
I know that Mr Harkness was not asking how long ago the phrase to make love acquired a physically sexual sense, but rather asking when it lost the possibility of meaning less. But since it depends so much on the user's intention, there are bound to be borderline cases at any time in the past 400 years. Even love alone has referred to sexual love ever since the Anglo-Saxons introduced the word to the island in which they settled.
In America they have a phrase to love up, which sounds very definitely physical - 'They love you up, your mum and dad,' as Larkin might have written. But one of the illustrative quotations in the OED has 'a puppy I was loving up'. Even in the Ozarks this must refer merely to a cuddle.