He mistakes her for his mistress and kisses her passionately. Since he is masked, his identity is concealed. Mistress arrives on scene, He realizes his mistake, feels terrible, and then forgets about the incident.

Fast forward one year, to the present, H offers for the dull but dutiful h, and she turns him down. Upon which, the H goes to soothe his ego with the same mistress (Mrs. Kitty Brownell). And the reason h turned him down? She wants the 'passionate' Lord Denham and not the bored/boring, uninterested one that's been halfheartedly courting her.