I think he might has been sad at the thought f Raeliana hating him if he continued to tease her; mixed with a bit of regret that he won't be able to hold things over her head to much, or it might lead to a repeat of the current conversation.
Oh I think differently, it look like to me Noah is flustered, he is sighing when Raaliana left. In our modern world Raeliana act when she cover Noah mouth is nothing unusual, but they live in those old fashioned world where noble and all that 'manner thing' still exist, so what Raeliana did to Noah, not to mention she was so close is a bit vulgar, she was a Lady after all, she is expected to be reserved and all, but she do that kinda thing nonchalantly. I bet no one dare to do that to The Duke (or maybe I am just a pervert LOL)
I'm not sure to be honest, the translation was off. I didnt know what to change it to sorry. I like shyshyshy's thought though!
Actually,your point made sense:)..Any explanation for my first question?
What did Raeliana mean when she said"As long as you know,we don't have to do this again"?Also,what emotion did Noah go through when she asked "Will you stop?"He kinda looked sad.