Proving that photons have mass would be a significant blow to relativity. I am certain that physicists have thought of this before, and that USC and Damo getting on a political message board one night aren't coming up with something revolutionary by claiming that light has to have mass to be effected by gravity.
It's affected because of the way objects with mass bend spacetime. The amount of bending in light is consistent with Einsteins predictions for how much of a bend this would produce. This was the first bit of confirming evidence for general relativity. I'm sure you guys haven't produced any alternative math for how it would be produced by protons with mass, so we're getting no where.