Weakness
He's one of those people that seem more robust without their clothes. Tailoring emphasised the slim shoulders; its absence revealed muscles unobtrusively defined. From the first night, when he'd seized her throat, she'd known that he was stronger than he looked. Stronger than she'd thought.
Or perhaps he simply upset whatever thinking on strength and weakness she'd absorbed from college hazings and the gladiator contest that was work. As for example: when she looked down at their entangled hands, and could tell only from the polish which was hers and which was his, she might once have found it funny.