Is the Fifth Dimension's "Wedding Bell Blues" autobiographical? Not in the strictest sense, since it was written by Laura Nyro, but it does feature Marilyn McCoo pleading "Will you marry me, Bill," and she did end up marrying fellow Dimensioner Billy Davis Jr. (The Fifth Dimension, I'm sure you know, were originally signed by none other than Johnny Rivers to his new Soul City label.)
But Nyro didn't write it for the Fifth Dimension; she wrote and cut it herself at the age of 18 in 1966. Three years later, Fifth Dimension producer Bones Howe suggested the song to the group, knowing that McCoo and Davis were indeed engaged at the time, but hadn't set a date yet. McCoo began acting out the lines about "Bill," which were in Nyro's original, in 5D's stage act, and the song took off, reaching Number One on November 8, 1969, and staying there for three weeks.
But by then, the song had already worked its magic. McCoo and Davis were married on July 26, 1969, and they remain so today. So in that sense at least, by the time the song was a hit, it was no longer autobiographical; she had married him, Bill.
Here they are, camping it up from late 1969 (I wish I knew which show this was from):