If he'd said, "I could beat up any one of you girls," I'd have conceded that he was probably right, and left it alone.
But that's not what he said. He said no girl could take down a guy, and that's so contrary to fact, it just seemed natural and obvious to put it to a quick, harmless proof.
I agree that it wasn't considerate of me. I agree that if he hadn't been a gentleman, he might very well have decked me in response. I only realized that after I saw his face-- my brothers and I would arm wrestle and play fight without it hurting their egos, and it just didn't occur to me that this guy would take it differently than my brothers.
(And, if you're the one I did this to, I'm still sorry.)