Who sang, "I'd love to turn you on," and what does it mean?

That is a line in an old Beatles song, 'A Day in the Life.'

John wrote the line, and it means neither have sex nor get stoned. Turn on means to turn onto other forms of consciousness. Before Revolver, John had turned on by using LSD, and he began writing songs influenced by the psychedelic experience. "I'd love to turn you on," which closes Sgt Pepper's final song "Day in the Life," is a heartfelt expression of respect for a person such that he'd like to help the person turn on to higher forms of consciousness with psychedelics. See also "turn off your mind, relax, and float downstream" in "Tomorrow Never Knows" at the end of Revolver.

It's indeed from A Day In The Life, but it was mistaken for a drug reference, as Paul stated in an interview it meant "Turn you on to the truth"