Theo's libido had been sorely neglected for quite a long time. Millicent had left him, hurt and feeling betrayed (not that he really blamed her), when Peter came along, and Theo had thrown himself into raising the boy and taking care of his family. He hadn't had any real time or inclination to find someone to date, and he wasn't the sort to just casually go out and find someone for a one night stand.
So yes, his mind was rather clouded and he wasn't thinking terribly straight. Later, he might recall that she had a secret fondness for romance novels. But later wasn't here yet.
She was just too damned good at being wickedly distracting, and he knew for a fact that he'd be reliving this encounter much later, sadly alone. He was tempted, so sorely, when she put her lips to his ear. All he could think about was how warm and attractive she was.
Theo couldn't help but trace a finger down her leg, over her robes, as she thought. It was a subtle form of retaliation and something he just wanted to do, damn it.
Though he was very surprised she managed to get it in one. "Well, well," he drawled softly. "Aren't you the terribly clever one." His lips curved up. "It was indeed the normally quiet Miss Brocklehurst."