the swept back wing is to make sure the tips stay out of the wake of the shock wave.due to the speed closer to the speed of sound. the faster the plane goes the more swept back it is with relation to the amount of lift needed.
A misunderstanding of the facts. The requirement is for the wing tips to remain within the shock cone produced by the nose, and the F-104 achieves this because the wing, although straight, is of short enough span to remain within the cone. The angle of the shock cone is related to the Mach number as explained in TyroPicards reference. It should be noted that the shock cone does not form until the aircraft reaches a speed of Mach 1, at that time the angle being 90°, so is not a factor in the design of sub sonic transports. Wing sweep is about controlling the Mcrit of the airfoil and its associated wave drag, in association with numerous other considerations the designers have to ponder to meet the design aims. One of the reasons, among others, the 707 could not be stretched like the DC-8 was because it had too much wing sweep.