Dear porfavor: The word "fly" for fold of cloth for closure of trousers originated long before
zippers were invented. My trousers had buttons until I was in highschool. The zipper is a wonderful
invention. Here is part of AHD about fly, pl flies. Note def. 2, sqq.:

1. The act of flying; flight. 2a. A fold of cloth that covers a fastening of a
garment, especially one on the front of trousers. b. The fastening or opening
covered by such a fold. 3. A flap that covers an entrance or forms a rooflike
extension for a tent or the canopy of a vehicle. 4. A flyleaf. 5. Baseball A fly
ball. 6a. The span of a flag from the staff to the outer edge. b. The outer edge
of a flag. 7. A flywheel. 8. flies The area directly over the stage of a theater,
containing overhead lights, drop curtains, and equipment for raising and lowering
sets. 9. Chiefly British A one-horse carriage, especially one for hire.