Katherine's boyfriend Anthony is a pilot, but it doesn't seem she's a big fan of flying. That's why his "Airplane Failure Proposal" didn't seem like the greatest idea.
He takes her on a romantic flight through the skies and suddenly something is going wrong with the plane making it nose dive. She's starts freaking out and finds out in the plane manual that he is proposing.
I'd say her face and response of "You're an Idiot" probably indicates she wasn't a big fan of the plan.