Mark seems to weigh the pros and cons of explaining the whole 'lip paint' scenario, but just drops it, focusing on the other part of Arthur's statement.

"I didn't mean to imply I didn't trust you. It's just it's... " A little bit of his own anger leaks away as he searches for the words. "It's a sensitive matter. And I guess it's not just personal, really, but I wasn't lying about it not being about you. But it has to do with more than just me, and so I can't just talk about it with anybody." He seems to realize what he just said, contextually. "Even... King Arthur, I guess."