You don't HAVE to do anything! The fact is, you lost credibility because you obviously can't support an outrageous claim. Yet somehow you think it's weird that I would want to see proof? If anyone makes an outrageous claim I expect to see proof, otherwise it's probably a load of B.S.. Perhaps my initial speculations rang true, forcing you to step back and better appraise your girlfriend's ass.
"My dick is 12 inches no shit!"
"proof?"
"how dare you!!!!"
Get it?