On some level, Sadako knew that Sweeney was exerting ownership of her. It should have bothered her, and likely would have if she hadn't suspected that it traveled both ways. By letting him own her, by being as devoted and in love with him as she would have been anyway, she thought she was binding him with velvet instead of silver. If she'd flirted with another man, or - God forbid - actually taken one to bed, she was almost certain she and that other man would've ended up with throats slashed more artfully than any of Sweeney's other victims.
But that wasn't something she was thinking about. She was thinking about how she was wobbling a bit dangerously from kisses deeper than any she'd had before. Her arms went around his shoulders, half for the thrill of contact and half for stability.