Historically, to make love to someone was to pay court to them, or perhaps to seduce them. If you encounter the phrase in the works of Anthony Trollope or Jane Austen, it has this connotation. An interesting transition is found in Dorothy L Sayers' the Five Red Herrings. Hugh Farren declares that he came home and found Campbell 'making love' to his (Farren's) wife. While Farren definitely suspects Campbell of the modern meaning, he certainly did not discover them in flagrante delicto.