If Penny wanted to believe she had been sober enough to deal with discussion with this sort of brevity, then Percy wasn't going to argue - it was water under the bridge, as far as he was concerned, especially in light of their present conversation. Percy had suspected Penny still had lingering feeling for him all along, thus the mixed messages he'd mentioned to her before. The same way Percy had lingering feelings for her, as displayed in his leaving Ron's party early. It wasn't a surprise. As far as Charlie starting a fight, that scenario had easily crossed Percy's mind, including a trip to St Mungo's, for Percy knew he didn't possess the same physical strength as his older brother.
"Then you've chosen Charlie," Percy simply said as a statement of fact. It was rather obvious, or Penny wouldn't be behaving in this way. "I'm glad. You've made the right decision." Again, not a flicker of emotion crossed his face - Percy was responding reasonably, but it was only to quell another conflict of emotions, within.