Walt was so astounded that he'd nearly dropped his mug of cocoa.  What had just happened?  Even as he tried to play back what had happened he couldn't begin to fathom why she was suddenly so sarcastic, or why she was glaring at him in such a frightening manner.  Walt slid the hot chocolate away from her warily in case she was suddenly--and unaccountably-- going to pour it on his lap.   He was honestly surprised his mouth hadn't dropped open, but since it hadn't his lips pressed into a thin line.

“Here I was going to snog the boy senseless but since you are now dictating the amount of enjoyment I can have, well I guess that’s all off the table.”

What had he said?  He'd told her to go and have a good time.  He'd said damn, because he knew he was slow moving, and a different guy would already have made sure she had Valentine plans with him.  He hoped she would not have so good a time that he missed his chance.  That's all he meant.  Apparently that was not what she heard, and as much as he liked riddles Walt didn't feel patient enough to work out the discrepancies.  Maybe if it had been earlier.  Or if he'd slept more.  Or...something.

"I--you--what?"  He was dictating the amount of enjoyment she could have?  Is that what he thought he did?  Was it the Prefect badge?  "I'm sorry, I didn't realize I was ruining your life like that.  If you want to go snog somebody senseless... you go right on ahead."