"She didn't mean to!" Instantly jumping to Euphie's defense because he knows it wasn't her fault and he's been doing his best not to freak her out about it. It's way too similar to her own mind control thing and he doesn't want her thinking about it like that.
"I told her it was weird, being in Naoto's body, and she said not to think about it and to enjoy myself so I..." He trails off, because there's no way he's going to actually be able to describe what happened, no chance the word masterbated is actually going to be spoken here. It's the sort of dots Jack can probably connect, though. "She tried to make me forget about it afterwards but it wore off. It's, um, I'm kind of freaking out about it, actually."