When your attempt to protect goes the opposite of how you expect.
Kuro: "Here's a Ring of Resurrection, just in case."
Everyone in Nightmare: you are filled with DETERMINATION
Sounds like they've taken away their "knives" so they can't "cut" you, so now they're gonna make due with their bare hands.
AKA your arm is gonna come off, would you rather it cleanly cut off or ripped off?
I think it's even simpler. As far as she's concerned there has been no stimuli so he shouldn't be hard. It's like if a woman was just bathing and was casually really wet (of course, this is a worse example since female arousal isn't as outwardly obvious but you get what I'm saying).
Introvert, my ass. He noticed and registered a stranger looking longingly at snacks and, of his own volition, went up to her and offered his. My introverted ass could never.
Rich girl lacking an understanding of what's considered normal, got here through special recommendation but does indeed work hard, and has her capabilities questioned (even by herself) because of said recommendation...
Welcome back, Momo Yaoyorozu.